Given the weapons and armor the guard and guard captain are using, I think they may have mixed up the art.

Bro I think your right. I just left a comment on the guard stats and came hear to see if anyone else has said anything 

Why do their weapons do so much damage?

That's a great question. What kind of longsword and javelins is this captain using?!

They probably have a similar trait to "brute" for bugbears. It's really just a way for them to make the opponents more challenging without giving them an insane number of attacks.

   So we're just...doubling the weapon dice of npc's willy nilly, with no explanation now? Cool. A few npc's like Bugbears had the Brute feature which did this in 5e, but it was rare, & clearly noted when they did. It's aggravating how in the process of " Simplifying" the game & shortening the text in stat blocks, they actually make it more complicated for anyone who wants to understand the math behind the numbers, instead of just accepting them at face value. 

I completely agree man

why is a guard captain stronger than a war veteran?

Glad it's not just me wondering why this guy has an unnamed magical longsword that does 2d10 damage as a 1-hander lmao

What happened to the Knight? This monster is so strong!

Oh who ever made this npc sheet cheated hard.  Not only do they get double dice damage on the damage rolls but this guard captain apparently has 3 arms ><.  One holding it's shield and 2 using the longsword with its Versatile  feature.

I see no problem with giving the monsters' weapons extra damage dice. It's just a way to make the math work better. Don't worry about it, and if the players have a problem, point out that monsters follow different rules than players because they need to accomplish different things.