That's a pretty extreme example, but it can happen if you have a disconnect and don't realize it. Here's a couple of articles that might help.
https://support.chess.com/customer/en/portal/articles/1444849-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken-
and
So at 9:05 this morning my ooponent had 26 mins left. Now at 9:30 they have 37 mins left. Why is the clock so inaccurate? This happens often.