alan_m explained on 18/01/2021 :
The boiler may be operating at the low burner mode for, say, half an hour
before the room heats up enough for the room thermostat to cut in. The
boiler, without warning, is told to shut down. The boiler throttles back the
gas by the largest amount it can by turning the burners off. Even with the
burner turned off the heat exchanger still is hot enough to raise a fast
flowing steam of water by 10C+ and if the flow of water is stopped by very
much more than 10C.

The only thing the boiler can use, to know to throttle back, is an
increasing flow (or return) from the boiler temperature - where the
room stat is simple on/off. Throttling has to work almost blindly.

My Vaillant, with its fancy control system goes one step further. It
knows the actual room temperature (or hot cylinder temperature) and
knows the desired temperature. From the two values, it knows how much
heat it needs to still produce, to avoid producing too much heat, so it
can throttle itself precisely to hit the requirement.