[Haskell-beginners] Why is the type constraint different from signature?

Hi All, can some one explain to me why ghci> let f:: (Ord a, Num b) => a -> b -> a ; f=undefined ghci> :t f 1 2 ghci> f 1 2 :: (Num a, Ord a) => a The initial type signature just required *a* to be a type that is an instance of Ord but after it had the additional constraint of Ord as well??? -- Best Regards, Boon Hui