It's a common cultural thing, I think, for women to casually note when another woman is attractive, without any kind of sexual intent. This isn't so true for men.
I think it comes from a couple of things.
First, we're often more aware of what's "supposed" to be attractive, because we're "supposed" to try to embody or copy it whenever possible, and that involves work most of the time. Guys, on the other hand, usually just go with what they've got.
Then, what makes up female attractiveness is more clear-cut than male attractiveness. For example, take the whole prettyboy vs. manly man situation: some girls go for ONE type, others go for another type, and so on and so on. There's a wide variation. On the other hand, while there are minor variations, there's pretty much one "type" for women. Skinny? Big chest? Check and check, that's it.
So if all else fails, you can pretty much tell what's "hotter" without even really caring one way or another. It's a simpler calculation.
So it's not about machoness or anything at all. ;P I just joke about the guys who make a REALLY huge deal of it, as some do. It's an "Okay, dude, WE GET IT" moment.