why does my clock run faster than my opponents' ?


Probably because the speed of light is faster than the speed of sound....that is why we all look a bit foolish before we actually sound like it.
Did you get it?
obviously it is a joke ...einstein said " everything is relative "
Your opponent is moving closer to the speed of light than you.
Exactly. Conversely, your clock appears to be running slower from his frame of reference. The maths get complicated, but so long as both players moves remain causally connected, then the order of the moves is still preserved, and that's the important thing.

oh, I see. that is because of interent lag on your end. It happens quite a lot. Try closing all other windows when you play chess.com. It also helps to only have 1 device conected to your internet router. etc.

It's a conspiracy, actually.
Shortly before you joined, there was one of your so-called RL friends on this board telling us he'd sabotage your clock and we should all pretend that nothing is amiss.
Needless to say, the whole site and every member immediately went along with the suggestion, and you're now the only person on chess.com with a faster running clock.
Life sure is tough.

I have this too, I mean - bullet it hard, but this way it is impossible if you're not Asian.
LOL Nakamura would agree

people will make fun of you for this frustration , but it is not always your internet connection don't forget the complex engines and data that is running on chess.com with so many others playing , over time chess.com has been able to study this data for there own uses and needs , after all there not just to provide the world with a excellent opportunity to play chess but to make money ..ask your self why would chess ,com want you to stay below a certian threshold , is thier a advantage to becoming a member? why would it be in thier best interests to keep players from achieving their best and shorting a clock time be an advantage? ...Is it possible to track successful wins and if a player is achieving better can data calculations be made to alter the outcome...yes ..

This has happened to me pretty much every game I have lost on time. Like you said, they "seem" to get extra seconds. I'll look at the board for what seems like 10 seconds, move, but it says I used up 40 seconds. It sure didn't seem like 40 seconds, but it was.
I guess the short answer to your question is because you use more time than they do.
Title says it. I keep losing although being much better because my moves use up more time than the others'. That's really annoying!