I think Michael's got it exactly right--it's to clear the carbs
I'm running Webers as well--only 40s, but I also have the brass nut beveled on the front carb, as the clearance is very tight because of the carb height and the manifold angle. I believe that 45 Dell'ortos are even a bit taller, as are 48 Webers (some of the later factory cars ran 48s); both of these probably need a bulge worked into the bonnet. If you look at contemporary photos of competition Fulvias, you'll notice that a number of them have a bonnet bulge to clear the carbs.
Ed