Joe is also shorthand for Joseph or Joel.
Just like Dick and/or Bob is for Robert.
Johannes would shortened with a simple "Jo" with no "e".
If people are using the male spelling for a girl name,
then they're going outside the norm to try and be original, which it isn't.
It's actually lame and lack of imagination.
Most folks in the more recent generations lack imagination for new unique non-stupid sounding names. So I wouldn't be surprised if more started naming girls with guy names and vice versa.
The moral of the argument is simple, we're pretty fucked as a society.