Because the users are not allowed to edit certain aspects of the forum software, only the Admin can do that and only if it's possible, I suspect not. Technically look at it as the date you joined the new forum, not the community. Date of registration of Trainz is in many cases different to the date of joining the forums, some join before getting Trainz some leave it days weeks months or even in a recent case a few years before joining the forum, some register Trainz and never join the forum, kind of impracticable to sort all that out for the thousands of affected pre crash Trainz users and get it correct.