T-stats are very flakey...
I learned the hard way to test every stat I install by boiling it on the stove and now I have the ability to verify the actual opening temp with a temp sensing scan tool. There have been several times when I thought I was "cooking" a 195 when it began to creep open at 160ish...or vice versa. The things are utilizing an ancient technology that is fairly reliable but not all that accurate.
From everything that I have seen, once a stat opens it never fully closes again until its wayyyyyy below its min temp...like near cold. Remember, t-stats only control the warm-up temp. After that, they have a meaningless existence as part of the machine.Once an engine reaches its min operating temp, it stays there with continous thermal energy thats constantly produced.