Live calculates the time it needs to process one audio buffer. This value is then compared to the time it takes to actually play one audio buffer.
Example: 50% on the CPU meter means that Live's processing of one audio buffer is as twice as fast as it takes to play this buffer.
Values over 100% are possible - this means that the calculation takes more time than it takes to play one audio buffer.
With this calculation, Live expects that the audio thread always gets the highest priority and doesn't get interrupted. The final prioritization of threads is done by the operating system, which means that other threads might interrupt Live's processing. This is the reason why other applications might cause CPU spikes in Live's CPU meter.
Another reason for differing values between Live's CPU meter and the system's CPU meters is that the Activity Monitor and Task Manager normally show 100% per core/CPU. For example, 200% CPU load on the system CPU load meter of a quad-core machine means half of the computer's processing power is in use.