I'd imagine the sound is more out of sync due to the component input. More processing involved in getting the image on screen and so lags further than HDMI.
To explain, flat panels all have certain amount of input lag, how much they lag between input and display depends from panel to panel. There's a certain threshold at which delay is noticeable, cross it by only a few milliseconds and something like your lip-sync issue becomes obvious.
When you use an HDMI connection you're sending a digital signal to a digital TV and so each pixel already has it's predetermined position, quick and easy for the TV to upscale (which still adds a modicum of delay on top of what the panel already introduces). But feed it an analogue signal and it will have to go through the process of converting it to digital, then assigning each pixel and then upscaling, which increases delay even further.
