[plt-scheme] Check Syntax question

; Be assured that I am very pleased with the tools like Macro Stepper and Check Syntax in DrScheme, but there is a tiny thing I do 
not understand:

; DrScheme, version 369.9-svn13apr2007 [3m].
; Language: Textual (MzScheme, includes R5RS).

(define-syntax a
 (syntax-rules ()
  ((a b)           ; *
   (list b b b)))) ; **

; When checking syntax I observe:
; * b highlighted as lexical var.
; ** first b highlighted as lexical var, second and third as symbols.
; An arrow is shown between the two occurrences of b that are highlighted as lexical vars.
; No arrows are shown from or to the occurrences of b that are highlighted as symbols.
; Same observations with syntax-case.

; Is there a reason for this, is this a bug or am I simply being ignorant?
; Best wishes, Jos koot

(((((lambda(x)((((((((x x)x)x)x)x)x)x)x))
    (lambda(x)(lambda(y)(x(x y)))))
   (lambda(x)(x)x))
  (lambda()(printf "Greetings, Jos~n")))) 
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