How to slice thin curved wall
Even though each path, in theory, is concentric around the center point of the arc, the pathing does not always work out that way - especially around corners and radii. 

While your 1.2mm walls should always allow for three 0.4mm paths, if the slicer rounds down the overall thickness to 1.19mm, it will not detect enough room for three 0.4mm paths - but three 0.39mm paths should still fit, even if the slicer determines the thinnest part to be 1.17 mm.