The Rulebook is defunct and quoting it, seems risky at best. I can see where the confusion comes from though.
EDIT: from the Official Rulebook on Netrep.net:
What is the restriction:
"During either Main Phase 1 or 2 of your turn, you can play (Summon or Set) only 1 Monster Card on the field."
What are the choices:
"When playing a Monster Card to the field, a player must choose to place the card in 1 of 2 positions: Attack Position or Defense Position. For Attack Position, place the card face-up and vertical (a Summon). For Defense Position, place it face-down and horizontal (a Set)."
Summoning:
"To Summon a Monster Card, select it from your hand and place it face-up vertically (Attack Position) on an open Monster Card Zone space."
Normal Summon:
"Summoning a monster without the aid of Spell or effects is called a Normal Summon. A Normal Summon can only be conducted once in a single turn, during either Main Phase 1 or 2"
Tribute Summon:
"A Tribute Summon is considered a Normal Summon."
A Set:
"To Set a Monster Card, select it from your hand and place it face-down horizontally (Defense Position) on an open space in the Monster Card Zone."
"A Monster Card on the field in face-down Defense Position IS NOT considered to be summoned. Instead, it has simply been Set and can be summoned with a Flip Summon (Flip Summon)."
I think based on that, it is pretty clear that they are infact different.
In fact the very first paragraph states: "1. Set or Summon Monster Cards" and while a Set is mentioned under Normal Summon, it is not infact a sub-category at all.
As well, Tribute Summon is specifically related to Normal Summon not Set, which is where the discrepancy comes from here.