Shabbos Bulletin Parshas Korach 5778

When there is an aufrauf AND a bar-mitzvah in a Shul on the same Shabbos, the minhag developed to give the bar-mitzvah boy maftir, with the chosson getting the Aliyah before maftir.

But the truth is that this minhag developed at a time when a bar-mitzvah was celebrated the Shabbos BEFORE the bar-mitzvah–and thus maftir was the only aliyah that the bar-mitzvah boy COULD get!