Name of POP account in the Finder

When trying to deal with an e-mail issue involving an upgrade from OS 10.4 to OS 10.5 on an Imac Intel i noticed a funny thing about the POP accounts. In [home>Library>Mail one user had "POP-(user name)@(name of incoming server)" whereas two users had "POP-(e-mail address)@(name of incoming server). ISP provider, domain name and incoming server is the same for all three accounts (users). Why is there a difference between them in the Finder?