How does 0.35mg of norethindrone prevent pregnancy when you ovulate each month but take the same pill daily at the same time with no missed or late pills in over a year?

The typical use failure rate is 8-12% and perfect use failure rate approx1.1%. There are other birth control options w/lower failure rates. Norethindrone prevents ( theorized) pregnancy by thickening the cervical mucous making it impermeable to sperm, changes the Fallopian tube undulations &changes the lining of the uterus thereby making implantation difficult. Failure is % women getting pregnant.