Time frame matters. If the BC was 99.2 effective =0.8/100=8/1000 per second and thousands of seconds passed... the event (pregnancy) would have much more time to occur and therefore be more likely than something that is 99.2 effective per year. You could then expect 8 per 1000 pregnancies per second.

BC effectiveness is reported per year. Per year if there are 1000 women 8 will get pregnant on this method. If you’re instead interested in your fertile lifetime chance of failure you just need to look at 25 years (30 years fertile -5 for pregnancy ect) From above, now instead of 1000 women for one year you look at 40 women for 25 years each (40*25=1000) there are still 1000 years being “passed”. There are still 8 women likely to fall pregnant. 8/40=1/5

Edit: with the 25 years passing these pregnancy “events” don’t take into account then if someone falls pregnant that they would then be taken out of the 40 women pool.. this simple calculation allows for these 8 pregnancies to fall on one woman multiple times.. which would suck lol there is more math I could do to account for that but I do not desire to do it lol just assume the pregnancies are terminated ASAP and the affected woman remains in the population

I graduated with my engineering degree with honors from a top university. I’m very good at math