That recessed floor under the flat sheet metal on the Suburban is what would be in the panel truck. It was part of a storage cavity under the seat platform. GM obviously didn't want to stamp out a floor for the Suburban and another floor for the panel.
The seat mount platform on the panel truck extends to the door, whereas on the Suburban it stops at the location of the jump seat.
That's why that thick gauge flat sheet metal is spot welded and bolted under the jump seat.