syntax - Head of a MatrixForm expression?

An example in http://www-zeuthen.desy.de/theory/capp2005/Course/hahn/mathematica.pdf amounts to the following

    mat = {{1, 2}, {3,4}};
    mat // MatrixForm
(* I've suppressed output *)

    Head[%]
(* List *)

Why is that head List rather than MatrixForm, in view of the following?

    Head[MatrixForm[mat]]
(* MatrixForm *)