My guess is it’s because an increasing number of women are repeat customers. The ones who are most rabid in the pro-life argument probably latch onto that 45% without bothering to break it down into how many of that 45% could be a result of failed birth control or whatever. There’s also the possibility that the number of repeat customers reported to the CDC are too low, as mentioned by beagledave.
Complicating matters are the few rabid supporters of a woman’s right to choose who claim to have had a dozen or more abortions. They’re either ignoring the dangers or are flat out lying to gain publicity, sort of the abortion version of Ingrid Newkirk.