I have the answer! I ride the same bike and tires year round on Boston area roads.
Cold effects bearings, tires, oil, O2 density, lungs, energy to heat the body and adds clothing weight.

You may have heard me and/or others say, "I don't know who is the person in the mirror; I haven't changed." Winter doesn't change our riding; it changes clocks, powermeters and odometers! It may even change incline on hills, length of roads and distance between mile markers! It clearly changes my bathroom scale.