Because PVA adds thickness which results in a not-as-tight fit as possible after it dissolves away. The wax works just fine. A semi-firm tap with a rubber mallet and the beams will release perfectly with a coat or two of wax.

Mike (the other Mike), I'm not sure I see the point of leveling the beams or even loosening the dolphin striker. I will take an "X" measurement on the boat to check squareness and try to do a little to square it up if it's out much...but you're not moving it much at all since the beam bolts are threaded into the hull. You have only the minimal amount of slop available in the beam through holes for the long bolt anyway..so most correction you put on it is just going to spring back after you release it anyway. Canted hulls introduce another level of difficulty (not the case with this F18 in question).