As I understand it the "Standard" load for the British 2" shell was 1 1/8oz. During WWI the load was legally restricted to 1 1/16oz. Following the war the slightly lighter load was so well liked that many shooters stayed with it & so it more or less became a Standard. The guns themselves though as Larry pointed out continued to be proofed for the 1 1/8oz load. The 2 3/4" chambered guns were proofed for 1oz.
That also fits in that the trend for very light guns really started between the wars. Typically pre 1914 guns are a little heavier than 1920s guns. I'm not sure either how much the change from black powder to early smokeless (nitro) powders may have influenced the load chosen.
