it's just her genetic background
waxing is not the answer for arm hair on females, though - it will always come back the same way - laser is the answer
with that being said, it's totally up to her - she's her own person and whatever she wants to do, or not do, is her decision
some straight men like females with arm hair (and even leg hair and a thick V bush), just like some straight women prefer very hirsute men
to each their own
however, irony would come into play if a female like Julia is only attracted to hairless smooth dudes