When companies put SA actions in LA magwells it messes up the CoG balance shooting off the ARCA plat or GC. So I'll bet that's why you are seeing this now where they are just building the magwell-shroud as far back as possibele (as you note, for the SA) and saying fk'it with how it looks on the LA. The industry used to often do it the other way, but people started complaining (eg Vision chassis Gen-1 as an example).
The LRH was originally designed for SA only. They had so many requests for LA they ended up inletting a couple and people were fine with the way it looked so they started selling them as is.