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I've noticed each member gets a different name when their post numbers change.

What are all of the names?
How many posts do you have to get to change the name?

you can change it to what you want yourself, after you've posted for a while.
go to 'user cp' then 'edit profile' then change 'Custom User Text'

I think it was 25 posts or staying here for 7 days...
can anyone confirm?

Its both the points mentioned by yers.