Relationship between EPC rating and primary energy use per m2

Other

Hello,
In 2018 I upgraded one rental for roof insulation and along with the existing ecotec boiler, double glazing (both circa 10 years old), LEDs, TRV, room thermo etc gained an EPC rating of "C" when it had a "primary energy use per m2" of 185 KWh/m2 per year.

In 2020 I upgraded another, this time with the same attic insulation (270mm) plus brand new ecotec boiler (Vaillant as before - "A"-rated), brand new A+ rated double glazing, LEDs, TRVs, room-thermo etc and as expected I obtained a lower (and therefore better) "primary energy use per m2" of 178 KWh/m2 - so far so good - yet I was given an EPC rating of "D"?

Both properties are of similar construction, but regardless of construction, how can a lower (better) primary energy use now give a worse EPC rating?

I have tried to discuss with the later EPC inspector, but he is absolutely clueless and just says "computer says D".

I assume there must be something else in the calculation other than the primary energy use per m2 - but have no clue as to what that would be and neither did the inspector.

Any suggestions?