Nima, when I'm home and thinking about it I'll pull out the data. But off the top of my head (and it's possible I'm misrembering the explanation, but I read it like 4x back when I was crazy TTC #1), part of the reasoning is that for most women, there are 2 days in the cycle where she has the highest chances of conceiving (I think it was 2 and 1 days before O, or either the day before and the day of O, can't remember which), and women who are having sex every other day are by default going to miss one of their two highest chance days, which becomes more important as women are older (the effects were higher for women who were in their mid and late 30s, as the younger women had a better likelihood of pregnancy from sex 3-4 days out from ovulation than did older women... in other words for older women it's more important to hit to best chance days of fertility). I think that's where the difference comes from, and if a woman generally had sex every other day, but for sure the two most important days in a row, then her chances were maximized.