ALSO remember..
Pregnancy is not instant.
The nite you have sex, you are not pregnant. Pregnancy takes up to 2 weeks to fully 'occur'.
First you have to ovulate, THEN conception has to happen. Conception can take 2-3 days.. THEN there is implantation, which takes at least 7 days...
So you see, pregnancy is not instant.
Taking the pill within an hour of when it is due keeps you at nearly 'perfect' use, which is the highest protection you can find.