A few thoughts...

First, how does the efficiency of your main speakers compare to the efficiency of your other channels? Less efficient mains may need to be set to a higher trim to match up with the other channels. If a previous calibration yielded settings of between -8 and +8, it's possible that this time you calibrated to a somewhat different point and simply shifted everything down about 5dB.

I hunted down the manuals for both amps on Rotel's site, and neither appears to have trim controls, so presumably there's no reduced trim on the 991 to account for the difference. What about moving the mains to the 985 and trying to calibrate? If there is some problem in the 991 that is causing it to need a higher level input signal, that might be a way to diagnose it (moving the mains to the 985 would then eliminate the 10-15dB difference between mains and other channels).