As others have said rubbish in, rubbish out. The program (I am sure) is sound, its the simplified assumptions.
The primary purpose of the EPC was for the government to get a survey of the country’s housing stock (probably an EU inititive) without bearing the cost themselves.
Whilst the aims...
The EPC is a tick box exercise very often carried out by people that do the course with no building / technical experience. It works well if you have a standard uniform box type house but introduce anomalies and extensions etc the assessor has to make assumptions which the computerised...