Tor, that is a good question and something I have wondered myself. In fact, I have several questions, particularly about the Winter as snow is an excellent insulator...
How much of a role does it play in retarding the growth of ice thickness over the Winter?
What is the year-to-year variation in that?
Is there a trend over the years?
Is it too cold over the Winter for much snow to fall, given that cold air can't carry much moisture?
Can enough snow fall on ice by early in the Winter to essentially block growth from then on in those places?
If snow falls, does it tend to become hard-packed and stay in place as an effective insulator, or does it instead get blown around into localised drifts?
And mainly to repeat Tor's question: how important is snow fall overall, relative to other physical effects, in the amount of ice growth and its variation?