I agree that the rating system is kind of silly. But it gives you a feel for how the Winter as a whole was, and its cool to see how the line graph rating the winter changed categories either way as the winter progressed. It rates the entire Cold season. Detroit easily rates as mild and we had 4 winter storms that caused damage. Meanwhile 2017-18 was rated as extreme and we had nothing damaging that year, just a good cold snowy winter.
You can check out buffalo's individual seasons here.
https://mrcc.purdue.edu/AWSSI/chart.html?stn=BUFthr
For detroit the most "mild" winter was 2011-12, just beating out 1952-53 & 1997-98. The most "extreme" winter was 2013-14. That was in it's own time zone, nothing else came remotely close. A far distant second was 1977-78.