Why do some painters leave a gap between the wall and the ceiling?
Surely this is down to personal preference, but here in the UK you might find that a room that has a picture rail painted in a similar manner.

The ceiling colour will be painted above the rail with a different colour painted below the rail.

One objective reason might be to hide imperfections in the join between the walls and ceiling. If the ceiling is uneven you'd get a wavy line around the room. Painting a stripe of the same colour around the wall could reduce the impact of this.

NOTE:


  Picture Rail Functional molding installed 7–9 feet above the floor from which framed pictures and paintings are hung using picture wire and picture rail hooks.