Computers dont always deliver the full power of Apples charger.
When I sync with my old white iMac, my iPad 2 doesnt charge (nor does it lose all that much). There just isnt sufficient power from the USB port.
Loss of 2% in an hour isnt critical; it would take over 2 full days to drain, and your synch will be done long before that. I dont think the issue youre seeing is the heavy load issue, which would be seen in high-end games, for instance (and is kind of to be expectedphysics is physics).
This is absolutely correct. Computer ports typically provide around 5W of power, whereas apple's power adapter gives 10W.