Just this morning, I had a thought on codex Parisinus gr. 1759 (14th c.) known as P.
Look at that manuscript, and it looks like the actual spelling of the word that everyone just translates as "seven/th". To me it looks like:
ευδομ(*superscript*) and not εβδομ/. Well, lol and behold, according to LSJ "εὕδομος" is Boeotian for ἕβδομος!
Henry George Liddell, Robert Scott, A Greek-English Lexicon, ε , εὐδιά-φθαρτος , εὕδομος
And Boeotia didn't use Gamelion as the name of a month. So, here's my scenario: Somewhere along the line, Apollodorus's work was copied by a scribe from Boeotia who didn't think his readers would know what Gamelion was, so he decided to put in the word "seventh" in his dialect as opposed to "standard" Greek to make sure to specify Gamelion was the seventh month.
All this is wild conjecture on my part, but I've seen academic theories built on less