unknown entities in the locale dtd files

Hi,
I am working on a UI testing application. I need to use the locale
translation files . I found out that some dtd locale files has
entities that is not in the "en-US" original files. I don't understand
why?

Here is an example:
Example file: l10n/en-GB/browser/chrome/browser/browser.dtd
lxr url: http://lxr.mozilla.org/l10n/source/en-GB/browser/chrome/browser/browser.dtd

This file define translation for some entities which is not in the
original file:
347 <!ENTITY spellAddDictionaries.label "Add dictionaries...">
348 <!ENTITY spellAddDictionaries.accesskey "A">

Original File: mozilla/browser/locales/en-US/chrome/browser/
browser.dtd
lxr url: http://lxr.mozilla.org/mozilla/source/browser/locales/en-US/chrome/browser/browser.dtd



Any Idea why?
I hope I am not looking in the wrong folders.