Check it again at speed. It's probably only doing it at idle, due to the sprung loaded non return balls atop the thermostat housing. one always opens a bit easier/before the other. once going (or revving in neutral) at speed the flow will open both sides. the two manifolds are fed via these spring loaded things, when one doesn't open much at idle, that side gets hotter, its no big deal.
"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013