Tldr: they're not pointing upwards, they're pointing to someone's head (the viewer of the painting/photography most likely)
I bet when they used one finger people were asking "me?" and then looked behind them to see if there is someone else. If priest uses two fingers it's obvious he's not pointing at you or someone behind you. It also helps if he points it little bit higher not just right between your eyes like a gun.
The Catholic Church has had quite a few years to solidify and formalize their rituals. Nothing a priest does during mass is accidental or coincidental. They go to school for these things.
so they retconned it?