Mine does the same thing. If you take a look at how that area is constructed, it makes sense. It's just a big, flat piece of fiberglass (and, as you pointed out, a larger area than your boat). There are no reinforcements on the back sides such as partial bulkheads or beams. On the outside, with the exception of right above the striping, there are no molded-in ridges or anything to add structure/rigidity. However, that's not to say that having flex is bad. Look at the immense amount of Offshore's produced and still in the water today. In fact, having some flex is a actually a good thing as it prevents cracking.