That’s exactly how it should be. The design is such that the edge of the blade rubs the frame and thus allows less detailing in most circumstances.
In technical terms an 18" squeegee is really 17.5" nominal. Like a 2x4 lumber is nominally 1 3/4’ X 3 1/2".
Unger ninja is a high quality product and if you take care of it it will last years of daily use and thus make you money.