If you are curious at how we determine a snowfall forecast, here's a glimpse.
Forecasting computer models output snowfall maps that show what they think will happen. There is clearly more involved, however here are some of the things we look at. The following images are from 3 different forecasting computer models with the legend on the bottom of each graphic.
First take note that they are ALL different. Each computer model (and us for that matter) has to account for timing, intensity, moisture, volume of snow growth region, and a host of other variables to come up with these expected totals.
This is why we forecast in ranges like 3 to 6 or 2 to 4, because it is impossible to account for everything. Not to mention, if even one of these variables is off, then actual snowfall will change.