There are two different/separate coolant temp sensor. One for the ECM and one for the instrument cluster gauge. The scangauge gets the coolant temp from the ECM. The instrument cluster gets the coolant temp from the other temp sensor.

EDIT: Added info. "FSM lists 2 sensors, one is located at front of engine next to the fuel rail cross overpipe ,this one goes to the computer.The second one tells the dash gauge the temperature, location not supplied by FSM.It indeed is at rear of engine as some have correctly implied,the main engine wiring harness is directly over it and surrounded by heater hoses and vacumn lines.I have only seen it thru a mirror! Thanks for the help" from this post: https://www.toyota-4runner.org/1830207-post13.html