The spell Twilight created was supposed to teleport her to Celestia to become an alicorn. But Celestia lost her book and had to use her phone to tell the spell to teleport Twilight. Problem is, autocorrect is still awful in Equestria and, when Celestia wrote out the spell, autocorrect mixed it up to one of her former students. Twilight Sparkle did not become an alicorn. Starlight Glimmer did.

This story goes off the theory that Starlight was a former student of Princess Celestia and abandoned her studies like Sunset. Whether you think it's valid or not, I still like it a lot.