It doesn't seem right. But it's minor in the scope of things.
That seems really bizarre to me. I would be really upset if I didn't notice and ended up with prototype or tooled part with the wrong wall thickness!
of course It doesnn't right ...
The working of three options (one side,two side and other) should be same,if we enter value say 10 mm thickness then in option midplane it should be 1/2 of 10 mm both sides from midplane.
It doesn't seem right.