[Bill Walker]

I think you hit a number of the key issues. One is certainly weight, of guns, ammunition and the structure to support the guns and the belts. IIRC Hurricanes had a truss type wing construction, with spars and ribs made of thin rod-like elements, while the Spitfire had a stressed skin construction, with stamped ribs and the spar being a mix of stretched sheet metal, extrusions and machinings. In general, a sheet metal structure will give you more choice in where you add new structural elements, which makes (again in general) for lighter structure.

The sheet metal construction will also give you more choice in where you place new items like cannon (easier to make cutouts, more choice in structural design of attachments, more open space), and therefore might have made it easier to stagger the guns and permit longer belts.

Also, you would need to know where the "other bits" are inside the wings. Studying the cutaway drawings on the Flight website might explain differences in the relative gun positions within the wings. I think both Hurricane and Spitfire cannon installations were done fairly quickly, so they would not have been interested in re-designing and requalifying flight control push rods and cable runs, fuel tanks, and whatever else was buried in the wings just to get a few more rounds in. I suspect that, in both cases, once they got a solution that worked, the design and test teams moved on to the next thing on the "to do" list.