The only thing I can think of directly off the top of my head is it's because the Bank 1 exhaust manifold has a much higher flow rate due to it's better design.
The front bank manifold design is big pile of crap! (Not saying the rear is great, just much better than the front)
You see the "common" code is the sensor is "below threshold" That doesn't mean the sensor is bad it's just that the deviation from sensor 1 to sensor 2 is not outside the minimum limits set in the PCM.
Perhaps...
The "threshold" should be been set differently for bank 1 due to it's better flow.
The higher flow is not letting the pre-cat scrub the exhaust enough. Especially as the precat ages.
The O2 sensor see's more unburnt fuel and just loads up faster and starts reacting slower.
I'll stop there. It's just a thought off the top of my head...
