What is the real accuracy of Seiko 6R35 movement?

Seiko only states the worst case scenario for their movement at the point in time it is shipped from their factory. As do any other movement manufacturer. No matter the movement you can adjust it to keep better time than the factory deviation (unless it comes adjusted from factory, then it should be pretty accurate from the start). Also the accuracy of most watches will drift over time so they should be adjusted regularly if you want it to keep perfect time.

Yea, Seiko is very inaccurate with it's accuracy informations.

I think it has to do with the legal protection against warranty obligations.  So -15 to +25 is not accuracy, but Seiko's understanding of when a movement is considered defective.  Many people who buy a mechanical watch for the first time do not know that a movement that is used for the first time takes some time before it has achieved the actual accuracy.  And initially, it can actually deviate by as much as 20 seconds or more per day.  Without this information, there would probably be many new customers who would give their watch back.  I think other watch manufacturers don't publish information for the same reason.  It would of course be different for chronometer-certified watches.

Thank you all for your comments !

About 6R35, the movement accuracy out of the box can be anywhere between what Seiko states on their website. But these movements if you regulate will run much tighter accuracy closer to +/- 5 seconds without any issues. Also, Seiko movements are very easy to regulate by any watchmaker or send it back to Seiko saying the accuracy is off. If Seiko regulates every movement at the factory, they may state tighter accuracy. You won't see most of the Swiss watches mention accuracy on their websites because they use movements based on the specific base caliber and different grades. The ETA2824 and SW200 accuracy is around +/-12 to +/-30 seconds, with accuracy varying based on the watchmakers regulation in the factory. 

The other factor about mechanical movements is new movements usually few weeks or a couple of months to break in and settle into a constant accuracy rating, it applies to other manufacturers movements too. Also, when the power reserve is low, the accuracy can deviate a lot and it is normal. 

My Alpinist with 6R35 has a trend that is undoubtedly particular compared to other watches. It is very sensitive to the position of the watch, especially the one at rest. Using it every day it is quite accurate because it tends to gain about +10sec during the night if in position with the crown downwards or upwards but during the day, by wearing it, it loses them and at the end of the day the time is exact. If I leave it resting face up instead it gains about +25sec during the night. If I leave it still for a few days, in that case it definitely needs to be adjusted because it earns even more than 1 minute. My other watches with Swiss or other Seiko movements have a more regular movement and gain seconds slowly, day by day, if they stay still there is absolutely no need to adjust them because they are too far forward. Reading documentation on the web and various opinions, I understood that this is exactly how it works.

I have all my NH35's and NH34's running at less than +-2 secs a day. I even have one that sits at 0.0