After some interwebbing, I found this info, which gives me a bit of an idea:
Causes go on to include running lean, egr issues, carbon buildup, and overheating.
You have indicated the computer is accounting for something, pulling more fuel to one side. That may be the indication you are looking for. Sometimes if injectors aren't flowing right, clogged, etc, it can cause this issue. It also indicates carbon buildup as well could cause this. If an injector is clogged, you will have one lean cylinder, and 2 rich cylinders, which will wreak havoc on emissions.
When was the last time you were on carbon patrol? I am thinking a good engine/fuel system cleaner (seafoam or techron) is in order, along with some spirited driving. If the issue persists, I would swap injectors from one side to another to see if the problem moves. Also look for vacuum leaks as well. Dryrotted vacuum lines are a possibility too.