If you try to run a wash along a seam or panel line and it's a flat surface, the wash will spread out and soak into the surrounding areas. Very similar to putting a drop of liquid on a paper towel, vs a smooth glossy surface.
I suspect that capillary action is drawing the wash into the pits and valleys of the relatively rough flat paint.
This can be helpful if you want to grime it up as Tojo72 mentioned.
On the other had, a gloss surface keeps the wash in the grooves of the panel lines and even if you do get some outside, allows it to be easily wiped off.