The idea is that the 103rd isn't a new regiment, but they are significantly less combat experienced than the 37th troopers, who are fresh out of heavy fighting with servants of the Ruinous Powers and have lost a good deal of their comrades. The 103rd troopers don't fit in, many of which are replacements and troops with little experience themselves (despite the relatively lengthy existence of their regiment), and while there are veterans among the 103rd, they have spent the last few months in transport ships and at munitorum staging areas while the surviving 37th were pulled straight off of the battlefield and have had little time to lick their wounds. 

Edit: I have internet again. Just waiting on Skwint's character and Jayne Marigold's backstory.