While trying to wrap my head around how type inference works in haskell

I stumbled across the following inconsistency:

If I type 

               :t (*)

in ghci, I obtain 

              

                (*) :: Num a => a -> a -> a

But if I write 

               let mul = (*) 

               :t mul 

I get the less generic type signature

               mul :: Integer -> Integer -> Integer

How is this inconsistency explained? After all, I would expect the haskell compiler to deduce the most generic type possible. 

Thanks in advance, 

Stefan.