Related posts:

https://www.reddit.com/r/AskHistorians/comments/1xdkih/why_isnt_edward_the_confessor_edward_i_of_england/ https://www.reddit.com/r/AskHistorians/comments/26u5hv/why_was_edward_plantagenet_dubbed_edward_i_when/ https://www.reddit.com/r/AskHistorians/comments/1x4hln/why_did_the_regnal_numbers_for_the_kingdom_of/

edit: I think that last link is the most informative.