Why is it that it is possible for a woman to get pregnant during periods?

Here's a thought...: A woman with a short cycle this month, say 24 days in her cycle, would ovulate around day 10 of her cycle. She tends to have week-long periods, with just light days at the end. She has sex during the 7th day of her period, and the sperm swim into her fallopian tubes over the next day or two, and some survive long enough to fertilize her egg that arrives on day 10 (3 days after she had sex).