I've just decided to keep it. No big deal, hasn't caught dirt or anything.
I assure you that if there is a small gap, there will be a small pocket of air. It may not be obvious, especially on a white iPhone, but it is there. I'll take a picture of one later.
That means your pieces were flush. When I applied my skin the top part had a very tiny sliver of air pocket whereas the bottom did not because it was flush.