The phenomenon is called a "breakthrough ovulation" when an egg is released despite the Pill's suppression. It isn't rare, and when it happens the egg can be fertilized, i.e. conception takes place, but the fertilized egg cannot attach to the lining of the uterus, which sheds it because of the hormones. Thus, yes, it is an abortion, regardless of whether the unwitting mother knows it or not. That's the bad thing, they never know, all these times it has happened but the women never knew.
Thanks for the information. My quick google of the term shows it's a phrase adopted by many on the pro-life side. I would be interested in hearing from some objective sources before I really buy into the idea. As a student of numbers, when you think of the odds of ovulating and then actually getting pregnant at the same time - it's very very unlikey.