Flirting. Through painful trial and error, I've found that my hunch that a woman likes me is almost always wrong. Someone will be flirting very heavily with me, and I'll think "there is no way in the world she's not into me", and then it will turn out she will not be into me.

Another possibilities behind this, in addition to Vladimir_M's excellent hypothesis:

There is a small percentage of women who look like they are flirting with everyone, even though they are merely being friendly.

If 5% of women are flirtatious with no attraction, they could still dominate Yvain's flirting experience if the base rate of attracted women flirting with him is low. Meanwhile, perhaps 100% of the women attracted to Yvain are a sort of serious or shy type who isn't into overt flirting. Then both P(attraction | flirting) and P(flirting | attraction) could be low.

In contrast, another man could experience that same 5% of merely friendly flirts, but also run into a greater number of attracted flirts, making most of his flirting experiences indicative of attraction.