You might have just answered an old query of mine. My left hand tank always empties quicker than the right if I leave on "both" in cruise - at any height. 172H. Glad to know that it's not me flying cock-eyed - although that could also be true
I've flown the K,M, and N plus a 182P which is basically the same system. I've always noticed that the left hand tank empties faster if you leave it on both. Probably why the POH certainly for the K, can't quite remember for the others. says to switch to one tank or the other for cruise.