Yes contraception does occasionally fail even when used correctly, but it's relatively rare. What is far more common is people getting accidentally PG when they were not actually using it correctly or 100% of the time. That's not actually a failure of the contraception itself though, is it? It's a failure to use it correctly or consistently. But statistically and somewhat weirdly if you ask me, that's still included when we talk about the various methods of BC and their efficacies in percentage terms.
There is undoubtedly a significant proportion of 'unplanned pregnancies' which it turns out are not the genuine accidents they are claimed to be and this thread alone is testimony to that. I think most of us can recognise when an 'accidental' PG resulting in the birth of a baby is truly likely to be accidental and when our intuition tells us it almost certainly isn't.
One way to tell the difference is to look at the outcome. Most genuine accidents, especially when BC was being used precisely because they didn't intend to become PG, end in terminations. If you are genuinely shocked and panicked at finding out your are PG then you almost always terminate unless there are genuine reasons why you cannot.
Depending on how soon you find out you are PG and what that does in terms of narrowing your options or making it harder for you to proceed with a termination, some women may find they completely change their minds about how to proceed and decide to have the baby even though there was absolutely no intention or secret hope of getting PG in the first place. This might happen after a lot of soul searching and weighing up of pros and cons. I know people this has happened to, one friend in particular who got PG in her second year of uni and didn't find out until she was five months. I'd have done the same in her shoes and I am very much pro-choice, but five months would be two months too late for me.
The women and girls for whom it's not even a question and they know from day dot that they will have that baby regardless of their personal circumstances or the state of their relationship with the father, they are the ones for whom these 'accidents' and 'birth control failures' are in all likelihood, not quite what they seem. That overwhelming desire to be pregnant often overrides all good sense. These girls and women know they aren't in the best position or the best relationship to have a baby at this point in their life, but they are determined to do it anyway, so they only way to bypass criticism or the concern of others is to pretend they didn't mean it to happen and to pretend they had to think long and hard about how to proceed and 'just couldn't go through with it.'
I'm sure with some very young women it's not even an overwhelming desire to be PG. It's just done as a lifestyle choice and an easy solution to being able to move out of the family home or the care system, or to be able to stop going to school or college without everyone getting on your case. Not having to face up to the very tough and competitive world of work, education, self-sufficiency and personal responsibility. It's easy to say 'I couldn't do A levels/go to uni/have a career because I became a mum unexpectedly when I was young.' It's a bit of a get out of jail free card.