The "Joined Facebook
10 December" isn't the date the creator of the page joined, it's the date the page was created.
What I think happened is:
jane doe creates a test page using her jane doe account before sandy hook. jane has no fore knowledge and the page is just a test and not about anything.
she gets the url facebook.com/pages/test-page/6587668768767
then sandy hook happens and for some unknown reason instead of creating a new page decides to just rename her test page and go with that. Unfortunately the test page although being renamed still shows Joined 10th December" and people freak out.