You try to decide for everyone on the planet then. Here are the choices I was given, but add as many more as you want:
1. they were carved over.
2. they were plastered over then carved over.
3. they don't look like air vehicles.
4. they were plastered over, carved over, then some of the plaster fell out.
5. they were plastered over, carved over, then all of the plaster fell out.
One person suggested they were somehow inspired by clouds or/and lighting. Even with all that my impression so far is still that they were carved to appear as they do which amusingly is something some people in this forum don't want to think about.