If you noticed, the text for the cards do not say the same thing.
Harpie Lady Sisters as you mentioned has the text "This card can only be Special Summoned by..."
Five-Headed Dragon has the text "This card can not be Special Summoned except by..."
The first case refers to how a monster must be initially summoned. Once you do that, the monster can be reborn from the Graveyard. (However if you just discard Harpie Lady Sisters from your hand, you would NOT be able to reborn it since it wasn't properly summoned).
The second case refers to the monster being a NOMI monster (a term I'm sure you've heard). In these cases, the way listed on the card is the sole way to summon it to the field. That's the difference.