Question about Rome Total War HELP?

When i first purchased Rome Total War about 7 or 8 years ago.I first played as the Julii faction.When i completed my first Imperial campaign.I remember ALL of the factions UNLOCKING by themselves.I remember playing the game for a few years with all factions unlocked.I later damaged my disc.So i stopped playing for a while.A few years ago i purchased another Rome Total War.When i finished my first Imperial campaign.Only a few of the other factions unlocked.I have tried to unlock the factions but cant seem to get it right.My main Question is.Not about how to unlock factions.But why did the first Rome Total War i purchased unlock by it self.Are they now made different ?Thanks guys