My first project was similar. I was syncing another audio track to a SW preservation and I realized that the video and audio, being from different masters, tended to go out of sync at the reel changes (about every 20 minutes). It’s because of missing or extra frames when they edit it together.
Additionally, VHS recordings, being analog, may be sped up or slowed down because of recording or playback equipment. It’s usually tiny, but it becomes additive over time.