Goblins, not hobs...

Today I got a slayer assignment of goblins. I felt kinda sorry for the little guys so I decided to go after their bigger cousins the hobgoblins. After I tracked down a few I spoke to Duradel to check on my progress and he smacked me! He could not understand why I went after hobgoblins when the assignment clearly stated goblins. Silly me; how could I have thought hobgoblins are a kind of goblins?