An old answer I saw said that Europe (where conventions originated) has a bigger lag in temperatures than North America has. We both lag and the lowest temps are commonly in Jan-Feb.
Two things because I'm not sure what you're referencing. 1) We inherited a shit ton of European norms and terminology. 2) The cold weather in Europe has much more lag because of their geography. Large bodies of water, which Europe is surrounded by on 3 sides, smooths out and slows temperature changes.