I imagine that a large number of these women didn't use barriers on the particular occasion on which they conceived, or that they skipped some pills or took antibiotics or herbs that counteracted the hormones, but the point is that encouraging people to contracept may not do much to prevent abortion. Condoms break and diaphragms slip. Also, the most popular methods have a significant chance of working after conception but before implantation. This gets them the name "contraception," but would do little to protect new little lives. Pills, shots, patches, IUDs, the devices women use to avoid the inconvenience of barriers or to avoid having to persuade their "partners" to use a condom, all fall into this category.
If one takes into account the 12%-20% "failure" rate of barrier contraceptives and assumes that a certain proportion of their users would abort when the contraceptives failed, one finds a fairly large number of abortions, even in people who are contracepting meticulously.