One possibility is that the train to Brooklyn arrives on the 9th minute, i.e. 10:09, 10:19, 10:29, etc. And the train to the Bronx arrives on the 10th minute, i.e. 10:10, 10:20, 10:30.

So if the man arrives between the 0th to 9th minute of any 10 minute interval, he ends up taking the Brooklyn train, and he only gets on the Bronx train if he arrives between the 9th and 10th minute.