elisp: apply variable or function

[Zhu, Shenli]

Dear all,

I am tracing the code in gud.el(grand unified debugger) and find code 
snippet below hard to understand:

(defvar gud-marker-filter nil)
(put 'gud-marker-filter 'permanent-local t)
... ...
(defun gud-marker-filter (&rest args)
  (apply gud-marker-filter args))

The local permanent variable and function have the same name 
"gud-marker-filter". But, why elisp engine *apply* variable 
"gud-marker-filter" but not function "gud-marker-filter"(C-h f tell me 
"apply function &rest arguments)?

Regards,
Shenli