First, the clock on the client is not the accurate clock, the one on the server is. Second, your opponent had 7.7 seconds to make the last 15 moves and many of those were likely premoves, which only take 0.1 seconds. Finally, your opponent had 3.5 second after their last move.
So, you were most likely seeing lag compensation and it appeared your opponent had less time than they actually had.
https://support.chess.com/customer/en/portal/articles/1444849-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken-
I played a 5 mins game vs, [public accusations not allowewmd -- MOD]. At the end he was able to make more than 15 moves with ONE second left on his clock!!! How was it possible???