Yeah I'm struggling to comprehend the OP exactly. Do you mean archers should get negatives due to seasons?

Withnar analysed it in this thread over on TWC:

http://www.twcenter.net/forums/showthread.php?p=7350815

Some arrow types do get negatives in M2TW if its raining/hailing/snowing.

Possibly the team amy decide that the rather poor quality northern european archers may be "snow proof" so their poor numbers do not drop further in snow because of their familiarity/adaption to those conditions. Would steppe nomads be a bit more "snow proof" than mediteranean/Iranian archers? Just askin'.