There seems to be some inconsistencies when implementing protocols that have multi arity functions depending on how the protocol is implemented. I have attached a clj file illustrating this. The short version is that multi arity must be defined as such w/ defrecord:

(defrecord Zomg []
SomeProto
(hello [_] 1)
(hello [_ _] 1))

And as such with extend-type

(extend-type Object
SomeProto
(hello
([_] 1)
([_ _] 1)))

I have only tested defrecord & extend-type. I am unsure how it works with deftype and extend-protocol.