CURA - shell thickness question

So, CURA has an option thats called shell thickness.
Ive read on many places over the web that the number of shell thickness should be divideable with your nozzle size.

My nozzle is 0.5 (ormerod 1), now, when I set shell thickness at 0.5 and print a wall thats 1mm thick i get 2 layers + some infill on some rounded parts.
When I set the shell thickness at 1mm i get only 2 layers (as i suppose it should be).

How come is that? If the wall is 1mm thick, shouldnt shell thickness of 0.5mm be enough for 2 layers one next to each other without any special infill?

Remember that there are two "shells" on a wall - one on one side of the wall and the other on the other side. So a 0.5mm shell on a 1mm thick wall will be solid (two 0.5mm shells on each side of the wall = 1mm). A rounded corner of a wall is not always a constant thickness, depending on how the model was made.

Also the horizontal shell will relate to a number of layers and so be a multiple of layer height, while a vertical shell consists of a certain number of side-by-side extrusions and so will relate to the extrusion width (NOT the nozzle diameter).

Dave