The weapon deals damage equal to your magic, (or maybe magic+1? I don’t recall.)
So what you are talking about is a difference in how the game calculates your current magic skill.
When you look at the list of weapons, then the damage displayed is ONLY your character’s base magic skill.
When you look at your hero within a team, you should have ALL magic bonuses applied - from classes, team bonuses, etc.
When you say that it is sometimes 5 and sometimes 6 - do you mean you actually see a difference in-game? That would surprise me. What I suspect you are actually seeing is the difference between a hero within a team, and looking at the weapon outside the context of a team.
Since you are a warlord, you have a base +1 magic when using red weapons. So I would expect that you see the crude club dealing 1 more damage that the card says when you just look at the list of weapons - because once it’s actually equipped your other bonuses (like the Warlord +1 magic when using red weapons) is applied.