[plt-scheme] Help with a Font Issue

Suppose you SLaTeX the following code fragment (with the ambient font
family set to Times):

(define (parse sexp)
  (cond
    [(number? sexp) (num sexp)]
    [(list? sexp)
     (case (first sexp)
       [(+) (add (parse (second sexp))
                 (parse (third sexp)))]
       [(-) (sub (parse (second sexp))
                 (parse (third sexp)))])]))

The - in the second CASE statement is set in sf.  Copying the
resulting code from the corresponding font-embedded PDF results in
something that *looks* on-screen like a "-", but does not match the
"-" you get from typing in that character on the keyboard.

Emacs does not manifest this problem.

For a test case, go to page 8 of

http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/PDF/plai-2006-01-15.pdf

and paste in the code, then the corresponding uses below.

Any thoughts or suggestions?

Shriram