The "theory" behind the cooler temps would be the exhaust flows out faster.
Kinda like a exhaust glows red on the dyno and does not on the road, the air flow across it cools the pipe
so in this case and again it is a theory is the slow lean and hot gas holds more heat in and letting it out faster would allow it to run cooler
I totally agree with the need for the PCV for the lean condition.