I had reason to change the email associated with my Amazon account recently, so now when I login to Amazon I use a different email and password but end up in the same amazon account.
When I use the very Amazon account details to login to the forums it sees me as a completely new member so I now have no access to all my old forum achievments or ability to edit messages etc.
Seems very odd that Amazon dont use your actual Amazon user identity for the forums but rather choose to use the email address of the account which can be prone to change at any point.