Makes no sense but in fact that is the way it is. Found a lot of stuff on the web and had a few friend with 2011 MBPs test it out and sure enough this is the case. 85W cannot power the computer under the heaviest loads, it needs help from the battery. It is a small drain but if you leave your computer to do some all night compressing you maybe disappointed if you battery is not fully charged at the start.
Weird thing is Adobe Media Encoder pulls more from the CPU 100% across the board and seems to do better, You still get some lose.
Along with Compress taking it into the “battery assist” mode running hulu and other stuff can cause it.
It is a little frustrating because this new MBP is now my only system. I needed to go mobile and had to sell my 2007 Mac Pro. Proformance wise I’m happy I just have to do extra planning to make sure the juice don’t run out.
The $64K questions is will it get worse when a Thunderbolt RAID, monitor and extra HDD are attached. This is a fun time to be running Apple products.