From what my searching gathered it's simply about typeface. Some typefaces have the "double-story" design "A" while some have the "single-story." I imagine the single-story "A" is done more when we're actually writing because it's a much simpler thing to write out. "G" is also another letter that gets this sort of treatment.

Apparently as far as the history is concerned, it's a matter of Roman vs Greek. Romans use what we commonly write(single-story) while the Greek used the double-story.

All of this doesn't come from any sort of expertise, just a brief google run. Searching "double-story a" will probably help you find more if you care. Make sure you quote it though when you search.