Cooler air is more dense and compressed. On the same boost settings you will always hit a lower peak pressure in extreme hot weather as apposed to the winter.
I know. I was actually saying that it was tuned that much lower for some reason. I went in for my "summer tune" with 32 and came out just over 28. This is the first summer tune where I didn't actually have the boost turned up.
I'll see if I can get my hands on the logs, fuel and timing maps. All runs are done on 93.