Face attribute value differs from definition

["Óscar Fuentes" <ofv@wanadoo.es>]

I have this on my configuration file:

(defface paren-face
   '((((class color) (background dark))
      (:foreground "#505a55"))
     (((class color) (background light))
      (:foreground "grey80")))
   "Face used to dim parentheses.")

(defun dim-parens ()
  (interactive)
  (font-lock-add-keywords nil '(("(\\|)" . 'paren-face))))

After invoking `dim-parens', "M-x describe-face paren-face" shows this

Face: paren-face (sample) (customize this face)

Documentation:
Face used to dim parentheses.

Defined in ‘emacs.el’.
        ....
       Foreground: #586e75
        ....


Note that the definition says #505a55 but the actual value is #586e75.
Why?

I'm using the solarized-dark theme. If I run emacs -Q and eval the code,
describe-face correctly says that the foreground of paren-face is
grey80.

GNU Emacs 25.0.50.27 (x86_64-unknown-linux-gnu, X toolkit) of 2015-12-30