Well, after a ton of searching, I couldn't find the answer.. how annoying.
But just looking at the man page for sar, it says that the commands cpusar and mpsar are basically the same as sar, but while sar reports activity on a single processor system, the other two commands report activity on multiprocessor systems.
It looks like cpusar will list individual CPUs and mpsar will average everything together..
Quote:
sar reports system activity on single processor systems. cpusar and mpsar
report activity on multiprocessor machines.
cpusar reports activity for an individual CPU specified by the cpu argument
to the -P option. cpu must be a number in the range from 1, the base
processor, to the number of processors configured on the system. cpusar
reports the data on CPU utilization, inter-CPU interrupts, and number of
locked processes.
mpsar reports activity for an entire SMP system by combining the data of all
the CPUs.
To me, that implies that, since sar isn't made for multiple processors, it probably just grabs the first one it sees, but I'm not sure..