M&B doesn't, it's microsoft's crappy coding to blame. Or do you really think that "system idle process" uses 100% of the system resources when nothings happening?
IIRC Windows reports CPU useage on a priority based system rather than actual useage of the processor - i.e. the app (or game) in the foreground will usually report around 100% because Windows has allocated as many resources as possible to the task. The game could be sitting there doing nothing, but the useage reported will be the same because those resources are still allocated, whether used or not (in CPU terms, Windows is reporting how many cycles the game could use if it wants, rather than how many cycles the processor is actually doing something).
You can get programs which will tell you the actual CPU useage of an app based on the instructions being processed, though to be honest it's not worth it. They'll cause a slowdown on the machine and won't provide you with useful information, unless you've a burning desire to know precisely what your processor is doing each cycle.