Active Mufflers

I have noticed that when I am driving in 1st-5th gears my AFR and Maf appear to be really good. I have stainless works long tubes and no cats with the OEM GT350 Catback. Shifting to 6th closes the muffler valves automatically.

once this happens I immediately become 8-10 percent lean (Per O2 Sensors). Doesn't make sense to me. Mufflers open in the same MAF cells and same SD cells and the AFR is +- 2%.

could the muffler back-pressure be effecting how the 02s read?

Bank 1 and 2 behave the exact same. I cant figure it out.

Whats happening with commanded AFR? I'm wondering if its programmed logic to aid in fuel economy.

The scavenging effect will be impaired with the increased back pressure. There will be less unburned fuel escaping during overlap. I don't think running a bit lean at cruise will hurt anything, especially since the cats are gone.