Every one of these I've seen is like this (including mine), I think that there is supposed to be a gap at the back of the skirt, look closely and you'll see that there is a thick(ish) foam padding in there and the gap is to accommodate this padding.
However if you really want to adjust it take a look underneath the skirt and where the fixing screws go through the holes you'll see that they are in fact longditudinal slots and are adjustable, it looks very easy to do.
Let us know how you get on.