My Rotel RSP-1066 pre-pro volume control acts V similar.
I don't think you have anything to worry about.

Volume controls have various tapers allocated to them. A linear taper will advance the level in consistent amounts with each consistent advance of the control. Audio tapers are designed around nothing specific. Some controls give large amounts of level change at the lowest portions of the control's rotation. Others place that quick change at higher portions of the control. Since the numeric readout has no real relation to actual level, your control is set the way Onkyo thought best. As long as you can reach adequate level before you run out of volume control, you have nothing to worry about only something to get used to.