Board Thread:Warhammer 40k General Discussion/@comment-24.177.35.35-20130916045011/@comment-7123746-20130916052410

Partially because many Imperial Guardsmen are conscripts- they aren't exactly well-trained, hard bitten soldiers right off the boat. They're often just scared kids that get handed lasguns and told which direction to point them... The ones that survive a while, however, well.... You've seen what death world veterans like the Catachans and specialized units of veterans like Gaunt's Ghosts can do...

Adeptus Arbites are more formally trained and tend towards being men and women who chose the career as an Adeptus Arbite. They also generally don't face down hordes of Tyranids or Chaos Marines on a daily basis.

In short, more suitable recruits, more formal training and a bit more margin for error give the rookie Arbite the advantage over a newly recruited Guardsman.