The newer low-dose pills are less effective at preventing ovulation and therefore rely more on the remaining two methods. As an egg is microscopic, it can be difficult to tell in any given case whether an egg really has been released. But Dutch gynecologist Dr Nine Van der Vange made an extensive study of women using these pills. Medical researchers simply don't know. That small probability adds up over many months of use.
Some people have moral objections to contraception in principle.
But that is not the issue here. Even those who see no moral distinction between preventing pregnancy through contraception and preventing pregnancy through abstinence, must still object to any birth control method that relies, even a small percentage of the time, on destroying a life that has already begun. I recently heard the interesting rebuttal that, as birth control pills usually act by contraception, that any abortions that result should be accepted as an unfortunate accident. This is a little like saying that it's all right to fire a gun randomly in the dark, as you usually won't hit anybody, and in the few cases when you do, it should be accepted as an unfortunate accident. The biology in this article is probably involved enough without going into a long discussion of probability.