[DRG]

Becasue the code is set up to naturally produce more deep snow in the colder months. If you increase the grass level in November , even though you may have it set to be a winter map what you will get is more light snow showing though tall grass. The entire map code is set up to produce slightly different affects based on time of year to maximize the variety of maps generated

The random map generate allows players to customize auto produced maps but that is in turn driven by the code that places more or less type of terrain on the map depending on month.. that's why autumn trees only show up in a narrow time span.