My understanding of it is that the % sign completes a mathematical equasion
example you have 20 on the screen and want to multiply by 10%
Action:
20 dividied by 100 (that makes 1%) = 0.2
0.2 then multiplied by 10 (because thats how many percents you want) = 2
Original number (20) multiplied (or added, or subtracted, or divided) by whatever the percentage was (2)
20 * 2 = Final number 40