Reliability.
Truck makers have generally always (detune) used lower hp numbers in the HD (3/4 and one ton) versions.
There also can be a lot of difference in the engines internals in some cases.
Example is GM 5.7 they used for years would get things like flat top pistons with a 4 bolt main with lowered compression in the higher GVWR trucks vs two bolt mains/higher compression and domed pistons in the lower GVWR trucks.
All done in the name of longer term reliability. What Ford does with the 6.2 could only be answered by their tech folks or a web for the 6.2 internals and spec sheets.
Ford gives the 6.2 17 different GVWR's from 9900 up to 14000 lbs in the F250/F350 SRW and DRW trucks.
