Why does my thermostat open in a pot of water but not when installed in my 1998 subaru engine?

My 1998 Subaru Legacy overheated. Upper hose hot and hard, lower cold and soft. Classic Thermostat. Replaced t-stat. T-stat did not open. Ran car for 2 hours at highway speeds without T-stat. No problem. Brought a Second new T-stat installed it, with water in the system and it worked fine. Ran it for 2 hours at highway speeds it worked fine. Replaced water with antifreeze T-stat did not open. I then heated the Original T-stat (the one that was in the car the very first time it overheated) in water it opened. I then heated the Second new T-stat (the one that opened then did not) and it opened. Both opened at aprox 170 degrees. I have now run the car without a t-stat for short spurts 10 miles or so with no problems. Why does my t-stat open in a pot of water but not when installed in my engine?