opponent won with extra seconds given by clock


This is called "lag" - the clock that appears on your screen is not exactly the "real" clock. Due to issues with connection and server and other tech-related things, if your connection is poor, then there may be a big difference between the times.
Often, I've had my opponent's time drop to 0, only to have it go up by a second or two - but I've never had it go up by 6 seconds.
*Note - if you play on your phone, then I'm not surprised at all - everyone has said that the chess.com app is horrific, with tons of problems.