This may have been due to the union of Shamash with the place where A was worshipped.

If, as seems likely, that near Sippar, there was another city on the other side of the Euphrates, forming a suburb to it (as Borsippa did to Babylon), the conclusion is perhaps warranted that A was originally the sun-god worshipped at the place which afterwards became incorporated with Sippar.[59] Such an amalgamation of two originally male deities into a combination of male and female, strange as it may seem to us, is in keeping with the lack of sharp distinction between male and female in the oldest forms of Semitic religions.

In the old cuneiform writing the same sign is used to indicate "lord" or "lady" when attached to deities.
Ishtar appears among Semites both as a male[60] and as a female deity.
Sex was primarily a question of strength.

The stronger god was viewed as masculine; the weaker as feminine.
Nannar and Sin.
Nannar, a reduplicated form like Babbar, with the assimilation of the first r to n (nar-nar = nannar), has very much the same meaning as Babbar.

The latter, as we have seen, is the "lustrous one," the former, the "one that furnishes light." The similarity in meaning is in keeping with the similarity of function of the two deities, thus named: Babbar being the sun and Nannar, the moon.