That is a little odd. It might be your headunit. What I would do is measure the ac voltage from your headunit at those volumes and record them down. Make sure you disconnect the speakers.
Tuan
It really is odd, I'm glad I'm not the only one who thinks that. It sucks too bein I spent just about $400 on this headunit. I'll get around to doin that, because it's a huge increase from 30 to 33...thanks