Are you using premoves? If so, every premove removes 0.1 seconds off the clock. It was posted here in the Miscellaneous section of the article.
Timer Seems Off?

I use them maybe ten times per game to start, however, that's a one sec difference. I am experiencing what seems to be 10-15 seconds. Either With me loosing or beating my oppenent by ten seconds. For example, I play this guy all the time and we always come down to the wire with about 50-60 moves a player. Now one of us looses while other still has alot of time on clock and in only 40 moves. Something changed and it isn't fun playing blitz right now.
Hello,
I play blitz chess and all week myminute games feel very different. It seems like one player or the other gets a slower move. Not sure if its the timer or server but something seems very off. I was able to make 60 plus moves in a good minute blitz but now I can only make 50 but 30 being the normal now. Did they make the moves take longer or something? It just doesnt seem balanced. Like your clock that game is either gonna be fast or slow. Thanks