Bi-metal thermostats, or therm-o-discs, are rated in a syntax of LNNN-nn ... where L is the limit-temp at which the heat source shuts off, and nn is the differential/drop at which the heat source turns back on. A high-heat bi-metal thermostat rated as L155-20, for example, turns the heat off at 155°F and back on at 135°F.
Dryers with multiple heat choices normally in the past had separate bi-metals for each setting ... 155°F, 145°F, 135°F, etc. Sometime in the evolution of engineering, maybe in the 1980s, an bias heater was added in some cases to apply an additional amount of heat directly to the bi-metal to "trick" it into shutting off at a lower air temperature, eliminating the need for multiple thermostats. High temp would not engage the bias heater, thus causing the bi-metal to operate at its "native" temperature. Lower heat settings would progressively apply more current to the bias heater to make the bi-metal effectively hotter than the air temperature, thus cycling the heat source off at lower air temperatures.
Appliance411 - Dryer Thermostats
An alternative to bi-metal is a thermistor (thermal resistor), which is an electronic device that varies in resistance on a curve against temperature. A control board reads the resistance and cycles the heat source according to its programming. Thermistors can hold temperature within a more precise range since they don't rely on a physical switch "flexing" to make or break the circuit.
What does a thermistor do in a dryer?
There may be a bit of temperature "overshoot" after the heat source shuts off, and a bit of "sag" while the heat source gets going. Clothes in the drum, particularly wet clothes, helps moderate the difference.
My dryer has a thermistor. The target temps are Low 127°F, Medium 140°F, High 149°F. I've checked it with a separate thermometer, and via diagnostic mode which reports the thermistor reading during operation. I ran it on Delicate/Low a few mins ago for 8 heat cycles with no clothes. The element relay clicked on/off on a differential of 10.8°F. The maximum "sag-time" differential I saw between the lowest and highest reading allowing for the element to heat up and cool down was 18°F.