I was playing 15 min game. I had 6 min left and my opponent 9sec. Every time he made a move his time limit increased. How come?

from what i can see from your games archive...you never play straight 15 min games...all the games with 15 minutes base time also have a 10 second increment (15/10) meaning evey move played you gain an additional 10 seconds...

Thanks for the explanation. I am new to chess.com and did not realize that this is what 15/10 meant.

 no prob...and welcome