(cons 4 5)
is
(4 . 5)
Officially, the printed representation of
(cons 6 (cons 4 5))
is
(6 . (4 . 5))
except Racket simplifies this representation to:
(6 4 . 5)
This is to simplify the printing. Without it,
(cons 1 (cons 2 (cons 3 (cons 4 5))))
would print as:
(1 . (2 . (3 . (4 . 5))))
instead of
(1 2 3 4 . 5)