In the development of Clojure 1.3 Rich wanted to add the ability for functions to return types other than Object. This would allow native math operators to be used without having to cram everything into one function.
The original implementation required functions that supported this to be marked :static. this meta data caused the compiler to produce two versions to the function, one that returned Object and one that returned that specific type. in cases where the compiler determined that the types would always match the more specific version would be used.
This was later made fully automatic so you don’t need to add this anymore.