Essentially in the lore the Arbitrators are a bit too high-ranking, well-trained and well-equipped to have them down fighting gangs in the underhive with shock mauls, shields and shotguns.* Various books say that, because of the different way the local populace is policed across the galaxy, there's often confusion between a local who is enforcing the law as a day job and an arbitrator. Arbitrators come from off-world scholams and answer to judges, so they don't fear local politicians, governors etc who might otherwise blackmail them. They are far more highly trained, drilled in the Imperial faith, held to high standards and represent the off-world power of the Imperium, as much to the local nobility as to the general populace as they can't be bought. Confusion also comes from the Arbites miniatures tooled up as riot police going way back too- some worlds have them in company strength with their courthouses as huge bastions and reminders of Imperial rule , but some might only have one individual, or just one that passes through every so often. Furthermore, the first novel about an arbitrator was called 'enforcer' too as far as I remember, adding to the confusion. Ultimately they haven't had a huge amount done with them as a faction so the lore has been pieced together over the last thirty years in odd chunks of background and varies by author.
I like the new models, and the nods to various Imperial outfits from other forces. I think this is also a push to finally separate the Arbitrators/enforcers look from 2000AD/Judge Dredd.
*Having said that, in a first edition Necromunda campaign played out in a hazy summer in the 90s, a friend of mine ended up a gang with a rating so high and so many skills, rare wargear etc that nothing short of a marine tactical squad could stop them...
