This is purely anecdotal, and not backed up by any studies, but I personally suspect that the 1%, etc error rates are accurate across the population. But not necessarily accurate for particular individuals.
If you have a bunch of sunflowers, grown in different soils, with a height of 60cm, 53cm, 52cm, and 47 more sunflowers with a height of 51cm, the average height will be 51.24cm. But suppose you have very fertile, well nourished soil in your flower bed and you buy a packet of seeds? You could be the one who gets the 60cm sunflower.
We're all individuals, with slightly different endocrinology. For example, some women have precise 28 day cycles, some have 35 day cycles, and some have absolutely no idea when they shouldn't wear white trousers.
My mother and I both found that breastfeeding suppressed menstrual cycles for about three years. (Dunno whether that's genetic, or because we don't drink dairy milk, which can increase ovulation, or a combination of both, or something else I haven't even thought of.) But other women can exclusively breastfeed and yet will ovulate less than six weeks after giving birth.
For some women, hormonal contraception suppresses ovulation perfectly (it seemed to suppress mine perfectly, which makes sense, considering how well breastfeeding does) but for other women, it obviously doesn't.