The additional uptake of sperm is only 5%. This might make a difference, but it might not be much. I'm guessing that the sperm in the first wave have the best chances, and adding a little more sperm at the back isn't going to do much.
It's not about the sperm with the best chances to fertilize, it's the sperm with the best offensive and defensive capabilities, relative to another man's sperm. Most spermatozoa are not even capable of fertilizing an egg; they are specialized "blocker" and "killer" units whose main job is to prevent other men's sperm from reaching the egg.
That extra 5% isn't about making a difference to whether she's fertile, but about her ability to choose which man ends up as the father, out of the various men she's having sex with.
Let's say she has sex with two men, one right after the other -- the one that makes her orgasm has a 5% larger army in the trenches, so to speak, which could easily be decisive.
(The book "Sperm Wars" discusses these and other evolutionary pressures on the orgasms, preferences, and genitalia of both sexes, in quite a bit more graphic detail than I think is appropriate for quoting here.)
>Elisabeth Lloyd: I don’t actually know. I think that it’s at a very problematic intersection of topics. I mean, you’re taking the intersection of human evolution, women, sexuality – once you take that intersection you’re bound to kind of get a disaster. More than that, when evolutionists have looked at this topic, I think that they’ve had quite a few items on their agenda, including telling the story about human origins that bolsters up the family, monogamy, a certain view of female sexuality that’s complimentary to a certain view of male sexuality. And all of those items have been on their agenda and it’s quite visible in their explanations.
>Natasha Mitchell: I guess it’s perplexed people partly, too, because women don’t need an orgasm to become pregnant, and so the question is: well, what’s its purpose? Well, is its purpose to give us pleasure so that we have sex, so that we can become pregnant, according to the classic evolutionary theories?
>Elisabeth Lloyd: The problem is even worse than it appears at first because not only is orgasm not necessary on the female side to become pregnant, there isn’t even any evidence that orgasm makes any difference at all to fertility, or pregnancy rate, or reproductive success. It seems intuitive that a female orgasm would motivate females to engage in intercourse which would naturally lead to more pregnancies or help with bonding or something like that, but the evidence simply doesn’t back that up.
The whole discussion. It backs my theory that using evolution to explain current traits seriously tempts people to make things up.