The snow amounts are dependent on the following things (in order of increasing importance):
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Settings within WXSIM regarding modification of intensity of precipitation, as per some settings under Preferences/Settings (fourth tab)
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Temperature and other factors affecting snow depth:liquid water depth ratio, compaction, melting, etc.
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The GFS and or NAM model data being imported.
To comment on each:
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These settings should be close to default values, and only different from that if you’ve done a careful study (over several months at least) to establish actual biases. This should be a non-factor for most people. In fact, the settings can be changed only in professional mode, because great care should be taken to establish that they would need changes. For most users, then, this is a non-issue.
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The algorithms here have been very carefully developed and are pretty widely applicable. Problems with this stage of the forecast process are usually minor, and there’s not much to do about them. It would account for slight differences here and there, but not huge errors or biases.
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THIS is the main issue. Snow accumulations depend critically on the exact path of storms. One a month ago here in Georgia gave me 4-5 inches, but 20 miles southeast of here there was practically non, and 30 miles northwest, they had 12 inches! Errors of 20-30 miles in storm tracks are common in all of the big models. In fact, that’s doing pretty well! WXSIM users almost always import GFS data, which is available worldwide and is one of the best around, though the European (ECMWF) model does somewhat better on average (it’s also expensive to get the data)! U.S. users also have NAM (North American Mesoscale) importable in the FOUS format. If both models are imported, they are generally blended by WXSIM, for a better result than either alone.
The NWS humans look at all these models and apply their judgement, hopefully adding some value to the raw products. In my experience, WXSIM’s output has been, on average, just about exactly as good as the NWS, but sometimes one’s better, sometimes the other. If you’re seeing big differences between WXSIM and NWS forecasts for this, it’s most likely (though I can’t say for sure) a matter of the ECMWF, UKMET, CMC, or some mesoscale models differing from GFS and NAM, and the humans giving those others more credence. Let’s see how it works out! 
Tom