why my opponent clock increases in time?

I noticed on several occasion just before "time out" my opponent clock keeps on getting extra time while my clock keeps decreasing. Is this really can anybody say is a fair game of chess?

You played 5 + 1. With every move both players will get 1 sec.

Thank you, it doesn't seemed that way, within few moves my opponent's clock jumped from 4:51 say to 6:15. May be lichess should show time spent per move, as I have seen it on other site.

There is something similar on lichess.
After you finished your game.
Click on "Analysis board" and scroll to the bottom of the pages.
Click on "Move times". And hover the mouse on the graph to see the time per move.

Thank you again, I noticed that 15 sec can be added but I do not really knows how it work, I had never used it. I do not know whether both get 15 sec or just one player, but when I was checking my opponent clock, his/her time was going up while mine going down.
I will check as u said.