E450 why is it lower towing capacity than a F450?

When looking at the towing capacity of a E450 cut away cab & chassis from what I can tell it's GVWR is 20,000lbs.

E-450 Super Duty Cutaway GCWRs: – 6.8L V10 and 6.0L Turbo Diesel V8 = 20,000 lbs.

Meanwhile the F450 listed at 28,000.

This is according to this brochure for the 2009 year model..
https://www.fleet.ford.com/resources/ford/general/pdf/towingguides/09RVTTguide.pdf

Anyone know if the E450 is actually only rated for 20k? Meaning less than even a single rear wheel F250?

The same type of brochure for my 2005 F350 SRW says my truck can only haul 11,500 which is not true because it's base weight is 8,100 and I'm good up to 25,999 in my current set up. Meaning I can have a total of 17,k between cargo & trailer..

How do you determine what the actual "real world" towing capacity is for something like the E450? I know it has to be more than 20k lbs it's a dual rear wheel truck, so how is it determined for the total weight capacity including the truck?

I know overall weights and trailer vs cargo and all that but I never can figure out how do you determine what the truck can actually tow real world not just what it's axles are rated at..

 

So do you have any idea how I'd find out what the total combined weight capacity would be? I mean I can't believe the total combined is only 20k when a F250 truck can have combined up to 26k with out getting into CDL.

At the very least I'd think a E450 should be able to have a combined towing capacity same as a F250 truck..

The reason I'm trying to find this out, is because I can score a very good deal on a 09 E450 with a 6.0. It's a cab & chassis ex ambulance and due to vans being taller I can much easier set up a "real" sleeper on the van chassis vs a truck. Then later on do a cummins swap and have a awesome pre DPF tow rig.. (the E450's with 6.0's never had DPF up to 2012 they were given a wavier for limited production because they needed them for ambulances)

I mean I can make a way better towing rig out of a van cab & chassis than a truck but the weight ratings screw it all up and I just don't get why.. I mean I can put a goosneck on the van cab & chassis just the same as a truck..

What I was looking to do is something like this set up..



I mean you can't tell me a set up like that is only capable of hauling 20k lbs combined

 

I dunno the truck is in another state, so I'm trying to figure it out based on Ford's released information.

Is it possible I'm looking at this all wrong? Could the GVWR be the 20k lbs then combined with say a 14k trailer it would be 34k combined?

What throws me of is the wording they used.. because they don't give a break down on the E450 like they do all the other vehicles it's just the wording off to the side of:

E-450 Super Duty Cutaway GCWRs: – 6.8L V10 and 6.0L Turbo Diesel V8 = 20,000 lbs.

Makes it seem like that's a the total weight rating combined, but maybe I'm just processing the info wrong?

 

unfortunately they've changed the cutaways to the newer sprinter style bodies that are more of a unibody set up and don't have diesels. I'm not sure if they even offer a E450 now as well.. or do you mean the dealer could give me info on the 09 capacities?