I would say with typical WWII equipment, under actual field conditions, CEP was probably somewhere in the 5-10 MOA range which would equate to a 10-20MOA across circle containing 1/2 the impacts - that is, 100y shots had pretty good hit rates, past that you started seeing misses. I know what you're thinking: "But this (insert WWII sniper rile) shoots 1.5MOA groups!". That's nice, but it doesn't matter. I'm talking about what can actually be achieved with all factors taken into account.
I don't know where you got your data, but I think that good shots, who I assume were chosen for the sniper role, can do way better than that. With the equipment of the day.
Using the CMP Vintage Sniper Match as a measure, all but the worst shooters or those with an equipment failure do much better than that. The match is set up to require identical rifles of that era with
somewhat relaxed optics requirements (Same power as original, make and model specified if not original). The shooter has 2 big, dirty, and usually wet sandbags to use as a rest, or can shoot prone with a sling. Targets are only exposed for 20 seconds per shot. All meant to at least simulate some of the conditions a sniper might encounter. Each team member shoots 10 shots at 300 and 600 yard highpower targets. The ten ring is 7 in. at 300 yds. (2.33 MOA) and 12 in. at 600 yds. (2 MOA).
Granted, ammunition choices are unlimited, and certainly more accurate than what was available to a WWII GI. A good topic for research would be "was ammunition other than M2 ball available to the WWII snipers"?
Typically, several shooters will clean one, if not both, of their strings of fire. You had better get scores in the low to mid-90's for both strings or you will finish near the bottom.
This years national match at Camp Perry is this upcoming Monday. The results will be posted on the CMP site.
I can't shoot in this years match. Do to an upcoming wedding, I can't make this years match, the first one that I missed since the inaugural in 2012.