> I'm not an expert on vinyl. The only record players I've interacted with have only had two speeds. Do most of them do variable spin rates?

Lots of turntables have variable speed, DJs can tell you more about it :)

> How frequently do the rates get out of sync? What causes it? Do they vary on an album by album basis?

When you're DJing, one of the main things you'd deal with, is making things be in sync, one way or another. I'm not sure how much value you'd get from an app like this that seems to be more "fire and forget" and not for on-the-fly matching.

I'm guessing this app is for people who do their own maintenance, repairs and such of turntables, so they can easily verify their work.