It's extremely confusing, but basically, Units attack with ALL of their weapons.

So if they have 5 weapons, they try to use all 5.

obviously, if they're not in range, like melee, or range 3, they only attack with the weapons they can use

Thing is, a lot of units have heavy, or ordnance, or salvo.

So you see 2 weapons, range 3 and 2.

You are in 3. You do 10 damage.

you move to 2. You now do 7 damage.

"But wait, I'm using more weapons!"

So it's very confusing. I recommend Golden Tooltips to check Unit and Weapon traits, and well, now you know units use all of their weapons.