PP needs to be cautious as some people think NFP is just looking at a calendar, or JUST temps. When done diligently, it's highly effective. Many people aren't diligent. When you are tta its best to err on the side of caution. Especially on a website
I've known people who "don't know how " they got pregnant because they were "pretty good" at taking the pill. If they forgot a day or two or 4 they would catch up. Or they were on the anti biotic. Or they went off for a month then went back on. Are those behaviors affect effectiveness.