broken timer
This could be an internet failure, or "lag restitution", meaning chess.com knows your opponent is lagging, and gives them refund in time. If you think this isn't the case, please report it using How do I report a bug or make a suggestion? - Chess.com Member Support and FAQs.
Clocks start moving on the client side as soon as the move is made but that transit time from your computer to the chess.com servers do not count against your opponent. The site also has some lag adjustment so as long as there isn't excessive lag, time will always be added back to the clocks after a move, though it isn't usually a lot. If you were seeing 2 seconds added, then likely your lag was higher, since that time would not count against your opponent.
https://support.chess.com/article/423-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken