This reminds me of how (American) football referees and their aides completely eyeball the marks until they matter – at which point they pull out the yardsticks and pretend to hold it straight. This is also why scientists sometimes use significant figures (although error propagation is better): extra precision means nothing. Try compensating for the wind, different splash shapes, the water gun's mark, and all its idiosyncrasies. It's hard.
Sticking out a water gun towards the target will "increase" range by the length of the humerus, which should be around 1 foot – remember, your forearm is already horizontal. Being a foot shorter than someone else will "decrease" your water gun's range by less than a foot – because the trajectory is relatively steep right before it hits the dirt. Combined, these can only account for two feet of variation. That's less than what we already see as a result of different bladder thicknesses, nozzle variation for homemades, or nozzle roughness if it's my XP 270. And since the difference in range between a mediocre SoakerTag water gun and a high-flow homemade soaker can surpass 30 feet, this variance really isn't too big a deal.
That said, some reported ranges differ quite a bit, e.g. the 15-foot-plus difference between iSoaker's and ZOCCOZ's ranges with the CPS 1200. I, too, am interested in what went wrong there, or if it all came down to things like dud bladders. But some of those suggestions sound pretty persnickety to me.
