I had exactly the same thought myself Sticks.
I don't think Federico's theory is correct tho, as in BB2 in particular, Helen would have gone rather than Penny, being the more conventionally attractive, bubbly one and therefore (according to Fed) more of a threat.
I think the thing the four of them have in common is that they were all maybe a little bit too forthright and open, and percieved as being a little "unhinged" or "in your face" when they first entered the house.
It seems the males can get away with that sort of behaviour, but it's the fastest route back out through the doors if you're a woman!
Of course the best way of ensuring you
don't get nominated in the first week is to keep your head down and not get yourself noticed - which very nearly backfired on Jon!