Interesting. Is this often what is used in other countries perhaps? I'm an American and I've always been asked in terms of inches. I'd guess centimeter would be used most other places.
I just got a haircut, here in the US they commonly use some unclear numbering system. They said to me "Do you like it at a 4? We could start there and go to a 3"
I looked it up and apparently they're 1/8ths of an inch so there's at least some sense to it.
In NZ, they ask what number, and it's a single digit, like number 3 or number 5. 3 is cut shorter than 5, but I don't know what the unit of measurement is