The O2 sensors are reporting the cylinder banks are lean, but are not causing them to be lean.
If the upstream sensor is reporting the bank is running lean, the computer will try to compensate by providing more fuel and richening the mixture. Upstream sensors will report that a bank is running lean when it doesn't 'switch' at an acceptable rate. If the computer continues to see a crummy switch rate, it will set the lean fault codes.
Since you have both banks reporting lean, you may want to check for an air leak on the intake manifold side of the MAF. Your upstream sensors are probably going out, and usually do after 60K miles or so. An air leak might also cause the engine to run rough but you may not notice it if it's slight and/or the engine's under load. Hope this helps.
Read all about it here
http://www.autoshop101.com/forms/h46.pdf
