That adds a lot of thermal mass as well.
One thing I've definitely noticed is that as I improve the insulation in one area of the house, other areas will get colder.
The reasoning is that the worst insulated spots in my house were near to the thermostat. Now they they can maintain heat decently, the further way areas are allowed to cool more before the heat kicks on.
The first tool someone should buy when investigating the thermal efficiency of their house is one of the cheap ($30- $50 if I remember right) infrared thermometers.
The temperature of your house is likely 5-10F off from what your thermostat claims, particularly the further you get from the thermostat.
Found that out the hard way when three years ago during our first Winter here, the water to the kitchen was frozen for most of the month of January. Thankfully our house has PEX piping, so no burst pipes for us.
Sealing and insulating those areas has made a tremendous difference, even in the interior areas of the house. Since the subfloor is wide open, a cold breeze from one spot can suck the heat out of the entire floor with surprising effectiveness.
You're correct that it doesn't make sense to insulate between floors, but you do need to insulate the entire exterior wall area, which includes the wall space between floors.