In real terms, because someone selling the product was on some huge bonus scheme for each product sold.
In reality, due to the refrigerant process, 1 kW of electricity was transferred into @ 3 kW of heating capacity. This would show a customer that they are @ 300% efficient. ( COP )
And the cost saving would have been displayed to the customer on those terms.
In depth reality, the heating by the refrigerant would only heat the cylinder to 45C and a booster element would be operated to heat the water another 20C to 65C ( COP - 0.6 -0.8 ) - not found in any documentation.
We have installed numerous units of this type over the years to replace electric hot water units - for a supposed cost saving to the customer on running costs.
My consensus on these units are, if the refrigerant side of the unit breaks down with in the warranty period it will be worth fixing under warranty. If the refrigerant side of the unit breaks down outside the warranty period - throw the unit away because the cost of repairs would far outweigh the supposed saving on electricity use.
I have no idea why you would install an electric storage heater on a property that had gas available.