Because some people like me grew up calling the "OCTOTHORPE" (it's actual name - not pound) a "hash". As far as I'm concerned a pound sign is "£" ... although I may be biased based on my country of origin. I'd never heard "#" called a pound until I came to North America. Why is it called a pound anyway ... you don't use it instead of "lb" or instead of "£", do you?
I've always used it as number ... as in 1¼" #8 screw.