In regards to the other women... it sounds to me like you are much more informed than the average woman about birth control in general. Mightn't it be true that they were less informed and so had no reason to suspect pregnancy simply because they were missing information that you have? For example, many women don't know that some antibiotics can affect birth control, or that vomiting/diarrhea too close to taking one's pill can cause it to be lost from the system without being absorbed (and if these were isolated incidents, they could well have forgotten them by the time they came into your office; heck, when I was a kid I temporarily forgot how I severely sprained my ankle, so I can definitely see someone forgetting they had vomiting or diarrhea once). In their mind there was no reason to suspect they could be pregnant because as far as they knew they were doing things perfectly. Of my friends I know things about HBC they had no idea were true, because they don't go to places like VP and they only go off what a harried doctor tells them, and I suspect that this is true for a large number of women. Informing yourself 100% about any method you used makes the "I was pregnant with no reason to suspect it" possibility far smaller.