“Does anything account for this being more common with girls than boys?”
A few factors do
Morrris often gets aliyas, he probably has a tallis bag he uses daily with his name Moshe. He might introduce himself to visiting Rabbonim etc where he might feel more comfortable using Moshe. IT isnt surprising that with time Morris shifts to Moshe
His wife Debbie on the other hand other than on her kesuba which she has tucked away somewhere likely never uses her name Devora. So it isnt at all surprising that they are now Moshe and Debbie instead of Morris and Debbie.
I would argue though that names like Gittel, Glickel, Fruma, Tziril, Faigie, Liba and even Sidney, Seymour, Morris, Debbie while not Hebrew names are all Jewish names