I also read it was something to do with VO2 Max and oxygen uptake/use...women also generally have smaller hearts/lungs.

Whatever the reason, they just are

I generally ride with blokes all the time (mainly MTB) and while there are a few that I can stay with (and annihilate downhill, yay!) its usually because they are not at their fittest, they put some miles in and they start to pull away again.

Not sure about road but on the MTB I think that the weight of the bike makes a big difference as well, my BF and I both have full suss bikes around the 27lb mark but he's around 50 lbs heavier than me. His bike is a smaller proportion of his body weight (coupled with the fact that he's as strong as a bloody horse) and he can just steam uphill in gears that would bring me to a standstill.