Why do some get extra time?

Why do some players get a 6second bonus every time their clock runs to 0 ?

Could be a correction for lag and it could be that the game's time settings have an increment added to both players clocks after their respective moves.

time is end and he still playing?!!!

Technically the time wouldn't have ended -- if it's lag correction it would have said 6 seconds on your oponent's computer when he made his move, but zero seconds on yours by the time the notification of his move made it to your computer some six seconds or so later.

but it keeps going until I am out of time...maybe 3 or four times.

Your opponent is likely seeing the same thing on their side with your clock only they're moving faster than you -- possibly even using pre-move.

there's lag on the chess.com server,  lag on your computer -- best is to have a wired connection to your DSL line, clear your cache, and don't have any other windows or programs running.    it is annoying during bullet games when you factor in time to your strategy.  you see an opponent go to zero, then go back up 5-10 seconds.  screws ups your game.

Nope that is all wrong, some players seem to get 1-2 seconds back for every move at the end of the game extending their time.  I've had 30 seconds + at the end of a game when my opponent has had 2-3 seconds - And I lose on time.......happened to me many times.

The site logs the moves on the games. If you look at the archive on the website, you'll see that they are not actually getting more time. As has been mentioned, lag compensation explains what is happening; the server is the official time keeper and the code decides how much lag is allowed and is corrected for.

https://support.chess.com/customer/en/portal/articles/1444849-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken-

I believe there have been some newer changes on how much lag is actually allowed, that isn't reflected in the above article.

 Yeah I've read that article.   But its not consistent with my experience.  If I've 30 or 40 seconds on the clock there'no way I should lose on time when my opponent has only 2 or 3 seconds left 0 and they still win on time.  At best its a false promise, I'm thinking well I've won this one on time for sure.  Only to get beat.  It means my opponents get 3 minutes 30 seconds or more to play to my 3 minutes.  When it happens i think the opponent knows exactly what they are doing.  Be interesting to see the balance between wins on time vs wins on checkmates....(NOTE: Preference is always to win by mate, not time).   Will check the times as you say.

Why do people keep resurrecting these decade old threads made by people who haven't been on for years?

Again, the server tracks timestamps. Can you show a game where you have 30-40 seconds showing according to the server and they only had 2 or 3 seconds? That would help determine if maybe the problem really was  a bug in your time display (either your or your opponent's time), whether your opponent was able to belt out enough premoves and quick moves to cause you to use too much time, or if maybe you had small disconnects that caused some time loss (they moved but your client didn't get in in the lag allowance time).

Half the threads on the forums are about "time cheating" and such nonsense on a bad day. Some people just cannot wrap their heads around simple concepts, like how a poor connection means their time has already started running down because the server has received their opponents move - even though it looks like they are still thinking.

See that paypal payment window to the left of the chessboard?  Select the amount of time increase and pay for it before your clock expire.