I'll have to explain it step by step so that you have no chance to misunderstand what I'm asking about.
First, what a convector heater is. It's a plasma TV shaped box usually made of sheet steel. There's no plasma TV in there, it's hollow. A set of holes is made in the bottom side of the box. Another set of holes is made either in the top side or in the front side near the top of the box. A resistive heating element is installed inside the box near its bottom. The box is either mounted onto a wall or installed on small wheels or legs. The heating element is connected to AC mains so that it can draw electricity and produce heat which heats the air inside the box and causes convective flow of air through the box. So it's yet another way to produce an old boring electric heater.
That was an unregulated heater which would either underheat or overheat the room most of the time. So a heater is usually equipped with a thermostat which is mounted somewhere on the box. A thermostat may have a rotating knob and a bi-metal plate or it may be an electronic thermostat which has some electronic circuits and button controls to set desired temperature. Either way it cycles the heating element on and off. So the heater is either fully on and producing heat or it's off and it's cooling down to room temperature.
Now welcome an "inverter control" heater. It's the same heater as above but now it has some clever electronic circuit that can run the same heating element at different power levels. I cannot find the details of how exactly it's implemented (and if I could I'd rather not understand them) but it sound very similar to how LEDs are driven with a switched mode driver - exactly the right average power is applied to the heating element and so it's always "30% on".
Then there're two very similar convector heaters - nearly the same shape and size, same power and both with "inverter control", produced by two different manufacturers. Some dealer claims that a certification lab tested both heaters and one of them turned out to be seven percent more electrically efficient than the other. I don't know what exactly "electrically efficient" means but the dealer frames it such that the "better" heater will consume less electricity for the same amount of heat.
How could this be possible? If it were LEDs than the driver losses would be useless (and even harmful) heat. This time heat is useful and it doesn't matter where heat is produced - in the heater or in its control unit which is attached to the heater. Either way it should produce one kilowatt-hour of heat for every kilowatt-hour of consumed electricity.
How could one heater be "more electrically efficient" than the other? Is it technically possible?