Other is on the iPad, not "in iTunes". (iTunes is simply showing you how the memory on the iPad is being used, it's an important distinction.)
Other is corrupt files, or anyhting else that the iPad cannot recognise. There is always a small quantity of Other, but 6.6GB is too much. Often a Sync with your iTunes Library will sort it out. However, you should note that while a Sync is taking place, the amount of Other can increase. On my iPod Touch, the Apps often get marked as Other for a short while during a Sync.
Personally, I avoid making changes to my Library/iPod while the Sync is taking place. I make my changes in the iTunes Library first, and the allow the Sync to copy the changes onto my iPod as a separate procedure. Otherwise, it's possible that moving media that iTunes is trying to move at the same time could cause errors.