The models with a capillary thermostat, I believe in most instances had a cam on the control that was notched at the marked positions of high, medium, low, and ex low for discrete temperatures .... it otherwise would be fully variable between the minimum and maximum, like a refrigerator or window air conditioner.
There was a revision for a short time, probably during the DD era, that had a thermistor instead of a capillary thermostat. I've never used one of those. Melvin has one I saw when visiting.