The time needed to deconstruct an UFO is buggy.
E.g. if it says it needs 1000 hours, and you look in 8 days later, its not 808h as it is supposed to be, instaed it is something about 900-950h.
I guess there is a failcalculation.
I didnt look into it, but the same problem might also exist during the production of normal stuff.