(symbol-plist nil) => not empty

[Kim F. Storm]

My emacs had been running for some time, and I did

M-x customize-group RET nil RET

Then I tried this:

M-: (symbol-plist nil)

To my surprise, nil had a non-empty plist:

(custom-group 
 ((nntp-authinfo-file custom-variable)
  (ada-tight-gvd-integration custom-variable)
  (breakpoint-enabled-bitmap-face custom-face)
  (breakpoint-disabled-bitmap-face custom-face)
  (nnimap-authinfo-file custom-variable)
  (nnimap-prune-cache custom-variable)
  (vc-arch-mode-line-rewrite custom-variable))

 event-symbol-element-mask (nil 0)

 event-symbol-elements (nil)

 modifier-cache ((0))

 custom-loads (ada-xref gdb-ui nnimap nntp vc-arch))

The customize things are variables and faces which
are lacking a :group specification.

I don't know about the rest...  Is this normal?