Why is Towing limit lower on Manual?

[BEST 4x4xFAR]

Okay,
So the Automatic is rated to tow 4,500 lbs, but the manual only 3,000 lbs. Same vehicle otherwise, so why the significant discrepancy in tow ratings? Is it because the manual gearbox is already marginal for the application (input torque rating of 225 lbs/ft, and the engine is rated for a max of 225 lbs/ft -- What are they doing for '07 manuals by the way)? 4,500 lbs is already low compared to its completion, but 3,000 lbs is just plain sad. Anyone know for sure why GM underrated the Manual by 1,500 lbs?