Tim, you are assuming all the fuel and oxygen you are putting in gets used up in the chamber. Misfires, partial burns, all dump "leftovers" into the exhaust side... along with your valve overlap ingredients (under the right flow conditions).
In my limited experience with my car and turbos in general, I could get my turbine to glow with lean mixtures AND with rich mixtures. There was somewhere in between that resulted in a cooler turbine housing. I don't have EGT gauges but I can tell when the turbine is hot by it's color, relative to how hard I had been driving the car. Not very scientific, but valid observations.
I also notice that when you put a "free flowing" exhaust system on a Z, you get popping in the exhaust system. Especially during decel.