If you log into multiple characters, each character adds a cumulative renown level so you can log 5 characters with 5 renown across your account and it will grant you the 25 renown achievement.
EDIT: It also grants renown achievements for factions you haven’t even unlocked yet - For example Assembly of the deeps.
EDIT: Character location and level don’t seem to make a difference, and still grants the increased renown (as far as the achievement is aware)
This unexpected behavior was due to a bug that we have now fixed. It should not happen again.
We will now create and deploy an adjustment for affected players. This will not be a step that we can take immediately, but once we’re ready, we will remove the errant renown and rewards.