I can think of a few things to check:
1. Its possible your temp sending unit in the thermostat housing is going a little out of spec, its fairly easy to pull out and test in a pan of boiling water (check at various temps along the way).
2. since the temp sensor resistance increases at colder temperatures, perhaps you have a little corrosion inline that is adding to the resistance and making the needle point lower than it should. Check the connector on the sensor for any corrosion.
3. Its also possible that the contact of the sensor with the ground supplied by the t-stat housing isn't very good. There's pipe thread compound on those threads, if there's too much then that could compromise the ground contact. A bad ground would also increase resistance, making the gauge point lower, but I think this one is probably more of a longshot.
4. check that the t-stat housing itself is tightly mounted to the frame to complete the ground connection, although that was probably just verified with the t-stat swap.
Or maybe the gauge itself has gone a little off. These bikes are getting old and some of the analog circuitry is starting to drift.
Mine has the symptom where the temp gauge stays low for normal riding, then when I pull up to a stoplight on a hot day the needle will move very quickly from the normal position to 12 o'clock. Then once it goes to 12 o'clock its a much, much longer wait for the fan to finally kick in, and all works normally there. I've replaced the t-stat and the temp sender unit, and triple checked everything else, and it hasn't changed, so I just live with it. It seems to have gotten hyper-sensitive to a very small temperature change for some reason.
So, you're not the only one with a weird acting gauge, check the stuff above and if you don't find any explanation don't worry about it.