I have figured it out. It behaves more like a balance control than volume level. If you increase one side, it reduces the opposite channel by that amount. Guess it prevents clipping, but makes it hard to get the relative levels right if using more than 2 channels. ?
If you measure all speakers first using an SPL Meter, then you can get relative levels correct. If you make changes on a speaker by speaker basis you will be chasing your tail.
Room Correction's volume level makes sure that there is always zero net gain in the digital signal. It sets the relative levels accordingly. For example, if your side surrounds need to be +4 dB and the rear surrounds need to be +2 dB, then it will do the following:
L, R, C, Sub: -4 dB
Side Surrounds: 0 dB
Rear surrounds: -2 dB
The new "Set levels from decibel meter" feature makes it even easier. Use Tools > Advanced Tools > Audio Calibration > Volume Calibration and measure the SPL at each speaker.
At the bottom right of Room Correction, click "Set levels from decibel meter" and enter in the SPL from each speaker. Now all levels will be set correctly.