Sat Dish - Focal Point

Hello,

I have a 144 inch dish, with 24 inch depth. According to the Focal Length calculation (D*D/16*d)
I should have a focal length of 54 inch.
However the current set up has Feed legs that mount the scalar rings to 58 inches.
A 4 inch offset is huge when dealing with microwaves.

I am scratching my head why it is 4 inches off.

Has anyone heard of this before??

Ps- sorry for the imperial units... the Americans have brain washed me