This is interesting. The only thing I can think of to explain it is, in a tank operation, your "hot" water will eventually begin to taper off in temperature. In a tankless, it will remain constant. Do you think that's it?
I'm not familiar with the internals on these, but I understand there are two different kinds: pressure balance, and thermostatic. In the former, when a nearby toilet flushes, and the cold water pressure drops, the shower valve compensates. In the case of thermostatic, the temperature is held at the set point by a mechanical thermostatic control.
I know the Symmons were popular in New England area, but I've used them in Michigan and seen them here in NC, too.