"Getting pregnant is a game of statistics. Two very fertile people who have sex together regularly (more than twice a week or so) have about a thirty percent chance of getting pregnant within a month. So if three fertile couples start trying at the same time, on average one of them is going to luck out right away.
Some people are less fertile than others. Maybe the woman doesn't release an egg every month or the man has a low sperm count. As a couple, they won't be infertile, just subfertile -- maybe they have a 10 percent chance of conceiving each month. And if by chance the man with the low sperm count is trying to have a baby with the woman who only ovulates twice a year, their chance per month of conceiving might be two percent, or something like that. But two percent isn't zero, and they might by chance conceive the first time they try; they would never even know they had a "problem"!"