My personal guess without checking any secondary sources would be that Sir Charles wanted to associate the rifle more with his "M10" sporter in .280, or at least have the same receiver markings on both for production reasons. The M1910 or MkIII action was designed around the .280 round, just as the later Patt.'13 rifle was designed around the .276 cartridge (which seems to have been pretty much a copy of .280 Ross) Ross hoped to sell his Mk.III/M10/M-1910 in .280 to the Britishicon and Dominion governments as the new "Imperial standard" to replace the Lee Enfield. There was about zero chance of that, but he was a determined sort of fellow and a Peer of the Realm...