Typical engineers dilemma, not enough room for 8, too much for 6.

Ford has a times tried to screw various industries.  They tried to screw the oil industry with F-type fluid, then later Mercon SP.  I feel like this was some kind of play to screw the wheel industry or something.  Frankly if I understood their reasoning I'd be worried.

There was a very specific reason for the 7 lug wheels.
In 1997 Ford came out with a new body style for pickups but only the F150 received the new body. The F250 and larger continued with the old body. There was a heavy duty F150 called the F150 7700 (it had 7700 emblems) that had a GVW of 7700 lbs. It is often described as an F150 body on a 3/4 ton frame. The wheels on this truck were built for a GVW of 7700 lbs and the F250 had a GVW of 8800 lbs. In order to prevent somebody from putting the "light duty" wheels on a "heavy duty" truck Ford used the 7 lug bolt pattern.
Why does nobody think a 5 lug bolt pattern is odd?
 

My understanding was that the frame under the trucks were the same for the 150/250/350.  This didn't change until '99 with the introduction of the Super Duty line, when the F150 became a different truck from the F250 on up.  The primary differences before that where springs and axles to better handle more weight. 

I'll also note that those 7 lug wheels were used on vans that didn't have any special badging.  I can't say for certain what the GVWR on them was as I never looked, but they were, I think, 15 passenger vans.