The cause of this is actually quite straightforward, and although you may not have noticed it if the LH edge of the top apron curls forward slightly the corresponding RH edge on the under apron usually does this as well.

The first pleats on both sides are the two which are opened out the most when walking and/or sitting (particularly if you use a type of chair where the pleats hang down on either side rather than being "sat on") - if these pleats do not completely return to the fully folded position then the "excess" material pushes the apron forward, thereby turning the forward edge out slightly - when standing try sliding your fingers into this first pleat and push the inside fold of the pleat fully into its proper position - the edge turn should momentarily disappear.

For some reason this seems to only affect the 13/14oz kilt, so it seems to be one or a combination of material weight, cut/shape/depth of these first pleats, how far down they are stitched, and/or the position of these first pleats relative to the shape of the wearer.

Unfortunately the cure is much more difficult - some form of adjustment to these two pleats should solve it, but I don't know enough about sewing to say how it should be done!