Samm,
I am just your average engineer, but I will try to explain it

It is just a number thing. You pick what ever unit you want to call 100%, if you exceed that unit, you have more than 100%
If your guitar amp has a rated output of 10, and you get 11 out of it, you are at 110% of the rated output. If you want to call 11 the max output, and you have it on 11, then you are at 100% of the max output.
Gunner