call me Cáitín
Didn't know that exact reason !Because they expand with heat and jamb, apparently a lot of the initial testing was done in the cooler months in the US.
I just though it was the plastic piston getting compressed with use, maybe add a bit of heat...
The Shimano pistons have 2 rings that provide the clearance, they are easy to skim a bees-dick off rather than the complete piston with SRAM.
First time it ever happened with a Shimano for me.
Taking the levers apart is also a lot easier with Shimano.