Temps differ for adjacent nodes

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Dear UMH folk, My EnergyPlus heating loop has two adjacent nodes that, in my mind, should have identical properties. One node is the supply side outlet node, the other is the demand side inlet node. I've confirmed via IDF review and SVG file that they are indeed adjacent ... but they have temperatures differing by several degrees C. That doesn't make sense to me.

Can one of you shed some light on why that might occur?

p.s., Simple, single hot water boiler loop with outdoor reset control