Lamiot

I am using Cura 2.5. The nozzle size is .4 and the wall thickness is set to .4. Cura calculates the wall line count to 3. I think the wall count should be 1 to give me a single wall of .4mm. When I try to change the wall line count to 1, I get a message saying it was calculated to be 3. I know a wall size of .4 is a thin wall and could give me problems, but why does Cura keep wanting to keep the line count at 3?