Litecoin has a super-stable difficulty, it's almost like bitcoin, most calculators work fine with it.

Verge actually should pay out BETTER than what the calculator says as the difficulty over the alast 24h and 7d is decreasing and the calculators always calculate a static difficulty and don't have any variance. AND XVG is actually vulnerable to a difficulty retarget exploit - it will retarget too low when hashrate drops under a certain point and then will retarget too high ... during that time you can mine much more profitable.. After that phase you stop your miners, mine a different coin and come back to XVG when the diff dropped again

Many other algos have the same problem on whattomine, simply try a few other pools and coins and you'll notice that as well, DASH is a good example - it was super-exploited with the D3's coming online back in the days.

However, to come back to the PPLNS vs. Prop - did you read my link and explanation about that ? Are you still so certain that Prop payout is so much better ?