I'm really confused.

Let's take a semi-hypothetical case. Say I have a house that needs 70K BTU/Hr to meet heat loss at coldest design temps.

The homeowner upgrades from 3 ton HP with 5KW heater strip to 3 ton HP dual fuel with 75,000 BTU 95% gas heater.

Below the set point, the gas furnace will produce about 71,250 BTU's/Hour.

But wait a sec. The old heat pump could only produce 36,000 BTU/Hour, and the 5kW heater strip 17,065 BTU/Hr. You can't really add them together, because when the heater strip comes on the HP is already running below optimal temps, and besides it is running a good percentage of the time in defrost mode, but if you DID add them together, you'd only get 53,065 BTU/hr.

Are contractors (in Maryland, anyway) allowed to undersize for all-electric installations, are all gas furnace installs vastly oversized, or am I missing something?