This is my opinion, and my opinion only:
I would speculate that the Church took these points into consideration. Fact is, the “abortifacient” action of the pill is virtually nonexistent. The pill basically stops ovulation, so an egg being released and fertilized is highly improbable. Even if that event occurs, it is not a given that the fertilized egg will not implant.
Furthermore, this event can occur naturally in the body and under similar circumstances. An egg can be released “by accident” and be fertilized, yet not implant because the body is in a similar state when taking hormones. The result is the same, yet one is called an abortion and the other a miscarriage.
I would speculate that if there was a high probability of miscarriage occurring, the opinion might be different.