If it's from Scarf to Scarves, why is it from Dwarf to Dwarfs?

It again involves the different phonetic values of the sounds of the same letter 'A' in either word, probably. The letter 'a' in 'Scarf' retains an open sound, so that the plural can take a soft 'v' sound to: 'scArv's'. But, in the singular for 'dwarf', the letter 'a' takes on more of a dull 'O' sound, so that the plural will have to come across as 'dwOrfs'.