Who? Ford Marketing department knows. I'd bet a dozen donuts that the payloads are lower because Ford Marketing wants to upsell the more powerful engines.
But, I neither know or care....not in a jerk way....but in a 'I don't hook up anything heavy enough to matter' sort of way.

 

If you can afford it, get the 3.5 or V8.

There's no mystery why the 2.7 is marketed the way it is - to help meet CAFE standards.

So if you want the least expensive truck, it'll have the least expensive, plastic oil pan 2.7.

If you want more muscle, go bigger.

If a .32 special or a 9mm will do the job, why have a .357 mag?

 

dealerships order more 2.7 Eco's to say hey we have this lariat at a cheaper price & better gas mileage, etc...
while it lacks in performance vs the 3.5 Eco or the 5.0
If you want a truck with good gas mileage & look pretty that hey you have a truck, then the 2.7 is for you.
If you want a truck to haul things then the 3.5 or 5.0 is for you

 

whatever the reason, the ratings are the ratings and shouldn't be exceeded no matter what. Most folks are like you, and won't hook anything up heavy enough to matter. But I had a nice chat with a guy at a local publix who saw my 3 kayaks on my tracrack. He has a 100lb boat he wants to bring with him, his 2 kids and wife in his 2.7EB with a 30' TT. I didn't have the heart to tell him he probably exceeds all his ratings because he thought it was a great upgrade from his Durango...….but shoot. nobody reads their damn owners manuals!

I'm on a social media RV group and they're all the same "our tundra pulls it like it's not even there". Well no, your tundra definitely feels it, but it has enough power, yes. Aside from blowing through your weight ratings with that 35' bumper pull toy hauler, i'm sure the power is OK...……...but once I start explaining how payload ratings work, and front/rear axle ratings, and how max trailer weight number from the manufacturer is meaningless for their loaded truck, they get totally silent because they have no idea what the hell they're doing.
Some of 'em get defensive and say they've been towing for 20 years and know what they're doing...…...but don't know what payload means. OR that the golf cart on the back of their half ton chevy with the 30' bumper pull blew through their payload and they've exceeded GVWR and GCVWR too...…...noooo.

 

I'm holding out for the 1.6L F150.

 

Lol
all the engines come with the plastic oil pan now. FYI

 

It'd be nice if they came out with a 5.0L turbo boost engine, that'd be impressive.

 

 

Here ya go...everything you never needed to know about Ford F150 frame options. From the OM, the ONLY current model that gets the HD/HPP frame is the 3.5 EB SCrew or the LWB SCab with Max tow.