How can my computer use 300% PC processing power?

On OS X, like other *nixes, CPU load is measured per virtual processor core. If you have multiple - you can have 200% and more combined under full load.

This is different from Windows and maybe some other systems, where CPU load is calculated per entire CPU (or CPUs - I do not remember how it behaves when having multiple physical processors).

OS X and Linux and possibly other UNIX operating systems (or rather the most common tools on these systems) display CPU usage differently from Windows. On Windows 100% means “all CPU resources used”. On Linux, it means that one “thread” (as in Hyper-threading) is completely busy. As such, it can reach up to (Number of Threads per Core) * (Number of Cores per CPU) * (Number of CPUs) in total.

Although not entirely related, another interesting fact: Windows 10 further normalizes CPU usage using the CPU’s current throttling state. So a 2 GHz CPU running throttled at 1 GHz will only be up to 50% busy.

As already pointed out in several answers, Mac shows the CPU percentage of each process as 'percentage of one core' so you should divide that by the number of cores [including HT as 'double']

If you're uncertain as to how many virtual cores you have, Cmd ⌘ 3 will bring up Activity Monitor's CPU History pane, where you can simply count the stripes ;)

Directly below where you chopped off the image is the information you need to quickly determine actual total usage for System:, User: & Idle: Therein lies your simple total to 100%