Not sure if this is all of it, but Roundtop will make as much as 6"-8" of snow on a very good night. That means no wind or a slight breeze with a W-NW component. That snow is beat down incessantly by the cats running up and down the mountain overnight. That process continues night after night, conditions permitting so that it's essentially forming a glacier.
To illustrate better - during a really good week of snowmaking when they can blow 24/7 they might make 50-60" of snow on select trails. When all is said and finished, that becomes a 15"-20" base. But that 15"-20" base is nothing like snow cover in our yards, it is packed and packed and packed until it's frozen solid. Then the groom puts nice veil of corduroy on top so the skier isn't sliding on ice.
Also, Roundtop benefits from having the vast majority of the terrain face north - they don't lose much on a sunny, dry day. High temp/high dew/fog/rain is what kills their base. Barring that they can keep going even in the skimpiest years of falling snow.