Hehehe... yes, it may as well be out of 20 if you use halves but then there is something sabout the human mind that likes things out of ten or one hundred regardless of the sense behind it
It is like a lot of game shows that always score in 25s and 50s. It's really the same as 1s and 2s but people like to see neat numbers that go into ten or one hundred, as I say.