The question of whether to give Abraham a rough or smooth breathing is difficult. Manuscripts differ. We could say that it begins with aleph and that aleph = smooth breathing. The problem with this is that it’s sheer prejudice. We don’t have data which show a regular alignment between aleph and smooth breathing. OK, you say, but rough breathing = aspiration, and we know from lots of languages, the original Hebrew, and the NT versions of Armenian, Coptic, Gothic, Latin, Syriac, etc., that it wasn’t represented by aspiration in those scripts. Fair point, but it still doesn’t settle the question of how it was (a) pronounced in Greek; (b) written by educated scribes.
Look at Liddell and Scott and you’ll see that, if you exclude alpha privatives, ἁβρ- (perhaps with the rough breathing representing Proto-Indoeuropean s) is a more common Greek word beginning than ἀβρ-, so why should Greek not use the rough breathing here and make Abraham sound a little more native?
What we’re printing here is not odd or a novelty. It was also what Erasmus printed and other early editions of the TR (which of course were closely based on the manuscripts available at the time). It’s also what you’ll sometimes find, for instance, in Niese’s edition of Josephus that Abraham is Ἅβραμος, given not only the nice Greek ending, but the nice Greek beginning of a rough breathing, in line with the mss.
So in making an edition where we try to model everything we can off the manuscripts we decided on balance to use a rough breathing for this name. It’s not necessarily a big deal, except that users may like to know that thought, care, and above all, documentary evidence has gone into decisions like these.