In areas where
Aaron is NOT a homophone of
Erin (e.g. the Northeast, as well as the non-American English-speaking world) there may be some other explanation for
Erin's use on a boy, but in the majority of the US, they do sound identical, so I don't think you need to look much farther than that. People hear
Aaron and
Erin as the same sounds, and unless they are familiar with and care about either the biblical character or the country, they may see them as equally valid alternative spellings. It would be about like choosing
Shawn over
Sean.