If any application really used 100% of the CPU then there would be no way for the operating system or any other task to interrupt it from doing so, since there would be no CPU time to give to the OS. You couldn't even move your mouse, let alone stop the game or benchmark.*
Those CPU load measurements are calculated differently across operating systems, and whether you're using 99% or 100% really doesn't matter. It's not like the OS cannot take "full advantage of the CPU"—that's a misconception.
* Of course, that's really simplified, not really looking at the details of process scheduling, etc.