Movie runtime randomly completely incorrect??

For some odd reason, some movies get a completely wrong runtime, usually under 25 min long when they are of course, usually 90 min or more. This seems to happen with +- 20,30% of movies and once I play them, even for a few seconds, then it gets corrected to the right length. I re-scrapped them all, and this time with imdb instead tvdb but it was exactly the same result…any idea what could be wrong?
Thanks,
Chumbo