I had a thought on this subject.
From what I understand, females grow slower than males. Perhaps when picking your slings (at a petshop or whatever) you might select the biggest slings, for whatever reason, which turn out to be male.
Now I have had MANY males spiders and bought them unsexed from very reputable breeders, so can't say that I picked the "bigger" ones or that they would blatantly only sell me males. For example, the 2 P cambridgei slings I bought last year, one is male and the other is female.
I reckon the 2 B albopilosum (of the 3) that is male was because when I bought them, we (myself and the store assistant) specifically picked the biggest 2 we could find, opened about 30 containers to check each ones size.