I've noticed something similar, if not exactly, to what you are refering. It may have something to do with your speaker calibrations. If you have increased the speaker output by X amount, the max volume output will decrease in relation to it. I don't know if its a direct relationship or not, I did not pay that close attention to it. When I ran multiple calibration tests setting the output level at 75db then 85db, I noticed the max volume level changed.
Hope that helps.