Under certain circumstances, a watch may not be as accurate after a repair as it was before.

During repair, the rate of the watch movement is checked using a timing machine and by observing its behaviour over a certain period of time. This usually takes place at room temperature and with simulated sequences of movements. However, on the wearer's arm the conditions may be completely different, which would result in the rate being different than during the test after repair.

The best thing to do is to look out for deviations and note down how fast or how slow your watch is each day. Your watchmaker will then regulate the watch according to the information you give him. Under certain circumstances, it may be necessary to repeat this step, but in the end your watch will be set perfectly to your personal lifestyle.

Anything that causes a loss of energy through the gear train to the pallet will cause the pallet to impart less energy to the balance wheel, this will make the balance lose amplitude. The loss of balance amplitude will mean the balance wil take less time to complete one cycle, therefore the watch will gain time.

Also anything that causes the hairspring to become effectively shorter, such as oil or magnetism that makes two or more coils to stick together will increase the speed of the movement.

The 7s26/7S36 is a Seiko workhorse movement with excellent reliability and used across the range of Seiko automatics. According to Seiko Corporation, the 7s26 movement able to expect timekeeping accuracy to +-15s/day. According to watch expert Mr. Ikuo Tokunaga, the 7s26 movement might get as far as +45 to -35 sec./day.

However it has been known that this movement typically runs fast for a few weeks when fresh from the factory. Also it keeps better time when worn on the wrist all the time. 

The 7S26/7S36 movement is a non-hacking movement that cannot be manually wound. It beats at 21,600 beats per hour, so a tad slower than a standard auto, which usually beats around 28,000 bph.

The fact that it cannot be manually wound is a drawback as it means if you’re not wearing the watch for more than its 42 hour reserve, you’ll have to rotate it to keep it going, or keep it on a winder, or reset the date/time.

Some users report that the watch will "settle down" to a more reasonable accuracy level after being worn for a couple of months.  So, please let the watch to settle down to a more accurate level. 

Meanwhile, please find some public reports about this 7s26 Watch movement:

SKX007 200m Diver Reviewhttp://www.geocities.com/watch_crazy/SeikoDiver/SKX007.htm

As for the accuracy of the mechanical watches:

http://www.network54.com/Forum/78440/message/1041036706/To+JohnsQ-©+As+for+the+accuracy+of+the+mechanical+watches.

Accuracy of diver's 7S26 automatic:

http://www.seiko-divers.info/scwf/index.php?mod=103&action=1&id=988060037

Hacking the 7S26 and other non-hacking movements:

http://chizeledlight.com/pages/horology/hacking-the-7s26.php

7S36 accuracy:

http://www.larrybiggs.net/scwf/index.php?mod=103&action=1&id=1100232592

Another review by a Seiko customer:

Initially fast 7S26 Diver:
I have one as well. When I got it first it was about 16 seconds fast a day, way too much for my liking. And gained further during night, depending on storage position.Details follow:My day is approx 17 hours long, wearing the watch whole day. I always take it off during sleeping, sleep period being 7 hours.Deviation as follow daytime first seven days: +15,+16,+11,+11,+14,+12 and +21 secondsIn adition came the night storage (position): Dial up +3, Dial down +3, crown up +4, crown down +8, crown left +11, crown right +1.As you see from numbers, daily gain varies, and I have notised it goes up when I am more active. (but not 3-4 minutes in a night). on the other hand, I do not jogg or dance. But I do swim and ski.
I tried to find a position to make the watch slow down during night with no success, it was stable fast.After a month like this I opened the watch and moved the lever slightly every day to make it slow down. Had to do it 4 times (once a day) before I was happy. Now it stays within 2 seconds a day, and every night I check with my radio controlled alarm clock. If Seiko is fast I place it dial up during night, it will loose 2 seconds. If it is 2 seconds slow, I place it crown down during night and it will gain 2 seconds. Result: very accurate, has been the last 3 months since I slowed it down.You can also see from numbers above that the fastest position is crown left (the position your hand would be especially jogging or dancing, depending on the dance ofcourse. I suggest to slow it down and then try to measure while jogging again. I think that the faster it is the higher positional and activity errors you get.

If your watch loses or gains a few seconds per day, you can correct it without expert aid.

We suggest the following methods to adjust gain/ lose time for watch:

Method 1:

The hairspring in the movement probably got stuck and caused the movement to gain/lose time. 

Suggestion is while you wear your watch and shaking your wrist at half the speed of the watches working frequency (3 times per second) for 5 seconds. Most customers found this solution working perfectly and now keeps time accurately.

Method 2:

The rate of a watch varies slightly depending upon its position. You can take it off at night and place it as follows:

Before a Seiko leaves the factory, the movement is tested in five different positions:

• Crown down
• Crown left
• Crown up
• Dial down
• Dial up

The movement is adjusted in each of these positions to an average rate of between 0 and +7 seconds per day. In other words, a Seiko should never be slow but may be fast by up to 7 seconds per day. The average daily tolerances for chronometer rates are between -4 and +6 seconds. This means that even a watch that loses up to 4 seconds a day can still be called a chronometer. 

The rate is also affected by the owner's lifestyle. The automatic movement will react differently when the wearer has an extremely active lifestyle from the way it would with a comparatively quiet person. However, any deviation from the tolerances mentioned above is not so much a question of function as of adjustment and wearing habits.

A watch may well behave differently after a repair.