This is because I suppose the simulator calculates the values based on the bogus accuracy values provided by the mod and displayed to the users (which do not actually correspond to the probability of hitting the target).
There are many cases where close range damage sharply rises (if it's possible, e.g. select training or starting stats for blops sniper). I don't think that coincides with what ingame interface says. Thus the gunsim has some additional distance-to-probability mapping, at least. And thus results are at least somewhat reliable.
It does account for accuracy and possible amount of shots for damage.
And there are some guns for which damage dips with distance (powerRangeThreshold/Reduction rules, wiki doesn't display it), they're rare though. E.g. - sonic guns. No idea if this is accounted for (probably not).
