I noticed it. I also noticed I didn't feel the same way others feel about it... being male in the head. I see everybody as the same, except women are more enigmatic to me. But I noticed they would gravitate to me based on appearance and I would somehow not quite know why, or how on earth to respond to it, except awkwardly.
I think it's a sort of acknowledgement of the shared experiences as women. Or maybe a sort of preference for female company. And I don't think men - as I don't - feel any kind of automatic brotherhood just on the basis of our sex. We can have a connection, but it usually revolves around something we have in common, some activity or something. But women can somehow hit it off very well very easily sometimes.
If I had to guess, I'd say it was something human females evolved over time to help them out building the different kinds of relationships and hierarchies among themselves than they would with men. Women seem a bit more co-operative on the whole than men, might have something to do with this ability.