Trailer towing capacity and GVW are both highly political numbers. They are a one part bean-counters covering their butts for warranty issues, one part actual truth, and one part competition.
A good example - my 98 F150 extended cab short bed in its configuration is rated to tow a measly 6500 lbs. If it had the single cab long bed, (same wheelbase) it would be 8000 lbs. Why? Probably because they assumed that people buying a long bed single cab would need more "utility" and those buying the ext cab wanted more daily driving. They could lower the tow capacity, save some $ on warranty repairs, and not really affect sales that much.