Adding internal partitions reduces heatflow to outside: why?

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Please see:

https://github.com/DamonHD/BuildingMo...

bungalow-2-dezoned.idf is a simple bungalow with 4 equal size rooms/zones, with partitions between them.

At winter design day, flat/steady temperatures inside and outside (ie everything is nominally in equilibrium) to eliminate issues with thermal capacity/'mass', heat loss is 1777W, less than expected.

I remove the partitions and merge the zones (bungalow-2-zoned-purchasedAir.idf), heat loss is now 2025W, more or less as expected.

Why the difference in heat loss when external surfaces and temperature/conditions are unchanged?

(I tried halving the partition wall thickness in case the heat loss through the contact areas between partitions and external surfaces was different, but no change.)