When you get down to U-values that low, the glazing conduction from purely inside/outside temperature difference is going to be insignificant in relation to the other parts of the energy balance. You’ll find things like solar energy that’s absorbed by the glass and then conducting to the interior/exterior will dominate the conduction term rather than this temperature-driven flow that you’re expecting to see here.
So you should see a more significant difference if you were to compare a U-value of 1 to 0.5 (IP). But it’s never going to be a perfectly linear relationship. Especially on a monthly basis like that where you’re averaging hours when your loosing heat through the glass with the hours when you’re gaining heat through the glass.