Classic benroy questions

While lurking last week I found the 77 page drawings and instructions for the classic benroy. Now, I can't remember where they are on the forum. Help a newbie.
I do remember the instruction to take 2 inches out of the width of the HF 5 X 8. What is the reason? Would I have to this with my NT 5X8 ?
Second, why build the platform as a separate all-wood assembly vs attaching 5/8 or3/4 ply directly to the frame?
Remember, I am a (newbie)^2.
Mike
I do remember the instruction to take 2 inches out of the width of the HF 5 X 8. What is the reason? Would I have to this with my NT 5X8 ?
Second, why build the platform as a separate all-wood assembly vs attaching 5/8 or3/4 ply directly to the frame?
Remember, I am a (newbie)^2.
Mike