This will not be exactly correct, but it should give you a rough idea of what's happening. I'll leave it to those with more technical understanding to explain it exactly.
It would be helpful to know exactly how your TV is hooked up to perform as a center channel. I can't see how you could get the center channel signal from the receiver to the TV, so I'm assuming you are simply using the normal stereo sound from the TV speakers as a pseudo center channel, and adding the sound from the speakers connected to your receiver to provide the surround sound.
If so, what you probably are experiencing is the difference in time it takes your TV and your receiver to process the audio signal. One is doing the processing slightly faster than the other and, to the best of my knowledge, there is no fix for this.
Edit: I sound like an echo!
