Situation: I'm on Norcal approach and they have already given me a squawk and altimeter setting. I am flying over KCCR where a couple of minutes later another aircraft is also transitioning over KCCR. NorCal gives them a different altimeter setting than what I have. Question is since my aircraft and the other aircraft are in the same area, shouldn't we both be using the same altimeter? I know I have been told to use whatever altimeter setting ATC gives me, but wouldn't it be logical to use the same one? Why wouldn't I switch to the other aircraft's altimeter setting?