Different people have different amounts of hair. I don't find it that common that women shave their arms though. I tried it once and it was painfully not worth it.
(Male model response): And yes, in some cultures (many, nowadays, given the spread of western norms, but definitely not all by far) females are socially encouraged to have as little body hair as possible. This is by no means inherently normal though, and in any case, therein lies the problem: you have not been exposed to the full range of natural female appearance - just to the de-haired appearance popular nowadays.