Knuth quote V

Somewhat mysteriously, in the middle of the chapter on macros in The TeXbook, Knuth defines \rhead—the macro he uses to keep track of the running headline. The definition itself is a little odd in that when \rhead is executed, it globally redefines \rhead to be almost the same text sans the definition.

\def\rhead{Chapter \chapno: Definitions (aka Macros)% my little joke
  \gdef\rhead{Chapter \chapno: Definitions (also called Macros)}}

However, it is the comment here that interests me. What is his joke? My best guess is that he used macros to change the running headline after the page on which this \def appears. He never calls attention to this change in the text and never explains the joke.