How can unplanned pregnancies happen if birth control is used regularly?

So I am 26 year old guy. I heard that some women have unplanned pregnancies but I always how that would happen. I mean if a woman uses birth control and never skips it for example if she uses the birth control shot or a daily pill or even an IUD and the man always used condoms, then how could unplanned pregnancies if all that done on time and regularly? I mean I been with my girlfriend for over a year and half and she uses the shot regularly and on time and I always use condoms and she never gotten pregnant. Also my parents, my mom uses birth control regularly and she only has me and never gotten pregnant for 17 years so far. I really don’t get, so unplanned pregnancies happen because some people miss using birth control? Is that why? Thanks in advance.