That's normal...more or less. Usually less in a perfect world...but...we're not there yet. Supporting the hull at different points will make it spread or narrow as the case may be. You might check that out. Hopefully it wasn't stressed in that way when you put the inside fiberglass on. Still it should spread a bit if supported on the ends. Sticks wedged in might help, temporary guide blocks epoxied on the inside and outside of the hull and deck might be an option. That would guide the pieces together as you lowered them together. Of course you would have to remove those after you have the deck firmly in place. I find it easier to work with a hull that is slightly wider than the deck, but what the heck. Strapping tape, or even duct tape, will do wonders when you start wrapping it around a recalcitrant hull. I'm sure someone else will come up with some ideas. You don't need to panic. There are always other options.