“How is 99% an order of magnitude different from 99%?”
OK, here’s how it works... With pharmaceutical birth control, there is a more than 99% chance that pregnancy will not result during the time that the female is fertile. A relatively a short window.
Using a rhythm method assumes you can predict the fertile period, and the 99% is on the rest of the month.
Apples and oranges.
BTW, I’m Catholic and have been married for 47 years. Been there and done that...
The “rhythm method” is no longer used. NPF is not this.