The T-stat doesn't control an exact temperature. As the coolant gets hotter, it opens more, perhaps going from full closed to full open over a 10 F temperature change. Under high thermal load (summer, driving long up steep grade), you need more coolant flow so the T-stat will be open more than in winter. To be more open requires a higher coolant temperature. Any engineer should know that as "proportional offset" in control theory.
Testing in a pot of water needs to be careful to draw firm conclusions. My 1984 M-B diesel car once was consistently running 60C when ideally 82 C. The T-stat seemed to work fine in a pot of almost-boiling water, but when I tested it against a new T-stat and some Mopar T-stats, it appeared to open sooner and respond more sluggishly. Swapping in the new T-stat made the engine run at 82 C. Too low an operating temperature will give poorer economy and more ring wear.
IR gun thermometers work best on a black surface. To get an accurate reading, the surface must fill the whole view of the detector, which I am guessing is much larger than the tiny laser spot. Note that laser is not involved in the measurement and is only for alignment. You need to get the gun close to the surface to insure that. Try various locations, like the upper radiator hose. Generally, you will read lower than the actual coolant temperature. Perhaps more accurate are the radiator caps with integral thermometer. That said, I was amazed how closely the IR gun readings were to a mercury thermometer placed in the hot water, even shooting thru the water at a shiny T-stat. The problem in-car is more that the surfaces you are measuring are actually cooler than the coolant, due to convective heat transfer to the air.
As a mechanical & aerospace engineer with multiple degrees and published papers in heat transfer and fluids, I should add that car hobbyists have strange and confused ideas. Many swear that adding a flow restrictor improves cooling. Their reason being that otherwise "coolant flows too fast thru the radiator to cool off". How do they come up with this stuff? A Robertshaw engineer also wondered and found the source of this myth (google). If true, then wouldn't the T-stat be working backwards (opens more to cool)? Another is that the T-stat is constantly cycling open and shut as you drive. I'd believe that if someone gets an endoscope video of it happening. I doubt Robertshaw would design their T-stats to exhibit "negative feedback instability". More reasonable is that it reaches a steady-state position between full-closed and full-open. Rant all you want, engineers will not entertain your unschooled ideas.
