It is said that the original 1962 recording of "Watermelon Man" by Herbie Hancock is in the key of F. It makes a lot of sense given it's a bluesy piece, so we have a typical I-IV-V based chord progression.
However, here is where it gets confusing to me: the first chord is F7, but there is no minor seventh in the F major scale. Why is it then that people still say it's in F? Wouldn't it make more sense to call it F mixolydian? Or is it implied?