I'm thinking - the online live chess programs take into account internet delays. While you're clock has counted down - your opponent may not have received notice you had moved for a couple of seconds. As such, the game adjusted the clock.
Were you face-to-face, there would be no question because you can see the clock at the speed of light. But this is the Internet.
I have just played player Samoan_Splash at blitz twice. The 2nd game I had about 19 secs in hand at the end and should have won however, his clock went to 0.00 then jumped back to 3.9 secs and he moved, I moved and his clock went to 0.00 agaon then jumped back to 3.9 secs again etc. until I had no time left and he 'won' by 0.9 secs.
What's going on????????????