Remember that the fans do not run when the car's speed is 45 MPH or higher. The thought is that at that speed, there is enough air flowing through the radiator anyway. Besides, the fan is wasting gas/power running when not needed.
At highway speeds, the motor's temperature is mainly regulated by the T-stat. Whereas at slower speeds, the motor's temperature is regulated by a combination of the t-stat and the cooling fan's set points. Further, the fan has two speeds depending upon engine temperature (and AC setting).
I see a couple of possibilities. If you are driving around town with the AC on, the fans run on high all of the time. This may result in more cooling air than the motor needs. Thus a slightly lower engine temperature.
On the highway, the fans don't run. The motor heats up until it is in balance with the t-stat opening.
I personally would not trust the dash temperature guage to make such fine determinations. Normally the anti-slosh circuits in the dash keep the temperature guage from changing very much.
So what does your ODB2 scanner tell you? What about the condition of the cooling system? Radiator clean? Anti-freeze/water percentage good? Is the pressure cap holding pressure? Does the de-gas bottle vent any coolant? Does the motor temperature continue to slowly rise on a long trip?
If the entire cooling system is good, then it is possible that a missing air dam is affecting the amount of air being driven through the radiator under speed. However, if the motor doesn't overheat on a long trip, it most likely is OK.
OBTW, are you enjoying your ride?
