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Q
I have seen this but do not understand it.
Oil-fill radiator electric heater is in a 10'x12' room =120sqft set at 71 degrees. The setting switch is set at 900watts. In a 10hr period it only used 5.7kWh! ( 5.7kwh is recorded on a meter)
The oil-fill heater is then set at 1500watts in the same condition. What is the kWh use? And why is it not (1500w x 10hr = 1.5kWh)?
What formula are they using?

It's unlikely the heater is running constantly in the 10 hour period to maintain 71F.  It should run until the thermostat is satisfied and only turn on if the temperature drops below the setpoint on the thermostat.

It looks like the heater ran for 6.3 hours in the 10 hour period, and consumed 5700 watts.

If you ran the heater at 1500W it would run less than 6.3 hours in a 10 hour period to maintain the 71F, because it outputs more heat than the 900W setting.  This changes depending on the outdoor temperature.

If the 1500W heater ran for 10 hours it would use 15kw, not 1.5kw.
1500 x 10 = 15000 / 1000 = 15