Why does a difference in OpenSCAD of two completely overlapping identical 3D shapes not give me nothing?

Question

I have been building a model in OpenSCAD where I have two hexagonal rings exactly on top of each other (one blue and one red). For each ring I create a cube and use a difference to mask off either the left half (blue one) or right half (red one). The idea is to have a hexagonal ring which is half red and half blue. If I do this with either red or blue it works. However, if I try both, what I see is whichever coloured ring is created last.

Perhaps there is a better way to do this but regardless I'd like to understand why what I am doing is not working. I've managed to get a much simpler minimal model with cubes which seems to do the same basic thing:

difference(){
    cube(side, center=true);
    translate([0,0,0]){
        cube(side, center=true);
    }
}

If translate is [0,0,0] I see a complete cube. If you offset them then you see what I'd expect as the difference between the two cubes. Why does a difference of two completely overlapping identical shapes not give me nothing?