I get this question a lot.
You're running a graphics program or some other application that is performing a complex operation that is taking several minutes, yes the CPU usage won't surpass one of these values.
If the value changes up and down, your application is probably I/O-bound, meaning the CPU is waiting for the disk or network to catch up before it can do the next step.
However if it is pegged at one of these values, it is because the CPU-bound operation is single-threaded, and you have a multi-code and/or hyperthreading CPU. In my case, I often see a process pegged at 12/13% because I have a 4-core hyperthreaded (8 virtual CPUs) i7-2600K processor, and the application has not or cannot split the operation over multiple threads.