It's a thing about Law, specifically the realm of Law, Mechanus, who's power I would guess they carry with them.

One of the (I think optional in 5e) realm rules for Mechanus is you do not make attack rolls. Instead, you treat any roll as a 10 and add your stat and proficiency as the realm removes the idea of random chance and focuses on clockwork regularity. Even damage die default to their average, which makes combat a lot more straight forward but also means you can come up on something you literally cannot hurt.

There's a handy common level magic item in Xanathar's Guide, called the Clockwork Amulet, which let's you skip an attack roll and just take it as a ten, and do so once per day (No attunement, resets at sunrise). It's a good thing in certain situations