Nothing will ever be foolproof that can be rendered ineffective by forgetting a couple of pills, taking antibiotics, or drinking grapefruit juice.
It could be that the pills are 98% effective only BECAUSE they can also hinder implantation.
BC Pills are meant to supress ovulation - one keeps one's eggs rather than losing them. Theoretically, the chances of ovulation occurring are already very small. Thus, the chances of ovulation occurring on the pill (regularly - obviously it's not fool-proof if you have to be responsible for using) and the egg being flushed out (i.e., not implanted) are very tiny.