This is just a theory: pheromones women release when they're ovulating. One of the few times I measured a full ten inches was with a girlfriend who told me after that she was ovulating. She wasn't on the pill so her biochemistry was au naturel.
Another time it happened was with a woman the night after a wedding reception. I think the whole wedding/reception experience causes a group pheromone release by the ladies present. The reproductive rite of a wedding makes some women broody. I have rarely been to one where I didn't get laid.
Like I say it's just a theory. It's not like I've measured myself in all situations or polled women about their reproductive status at every hook up. Those few events where I did get measured are, well, memorable.