Yesterday on UA 988, FRA-IAD, my wife and I watched the same film, started at roughly the same time (within a second--don't ask why, but she wanted to start them simultaneously). By the end of the 2+ hour film, we were off by over a minute between the two screens, with hers running fast.

How does that work? I noticed that PA announcements didn't stop/start the two screens simultaneously, but there were only a few of those throughout the film, and we're talking a second or so per announcement. I'm baffled how video playback would differ that significantly, although I suppose it's less than a 1% difference in playback rate between the two.