I'm just going by the resources I've used. Really, my only contention is that changing the rating system doesn't change the machine. If you bought a brand X model Z heat pump with a 20 SEER rating and their new rating shows it with an 18.5 SEER2 the unit hasn't changed, just the rating system.
But regarding COP and SEER, this is what I found, FWIW -
The SEER rating is calculated by dividing the total number of BTUs (British Thermal Units) of heat removed from the air by the total amount of energy required by the air conditioner in watt-hours. The higher the ratio, the more energy efficient the air conditioner is.
And this -
There are four heat pump performance measures: coefficient of performance (COP), energy efficiency ratio (EER), seasonal energy efficiency ratio (SEER), and heating seasonal performance factor (HSPF)
COP is the ratio of the output power to the input power, that is,
(COP=outputpower / inputpower
COP, which is a dimensionless number, is therefore a measure of efficiency; the higher the number, the more efficient the heat pump is.
The EER is the ratio of output cooling energy (in BTUs) to electrical input energy (in Watt-hour).
EER=outputcoolingenergyinBTU / inputelectricalenergyinWh
Therefore, it has dimensions of BTU/Wh. It is also noted that it is measured over time.
The SEER is the ratio of output cooling energy (in BTUs) to electrical input energy (in Watt-hour). SEER is a measurement of how the system behaves over a season where the outdoor temperature fluctuates.
SEER=outputcoolingenergyoveraseasoninBTU / inputelectricalenergyoverthesameseasoninWh
HSPF is a measure of the efficiency of a system and the units are the same to that of the SEER. HSPF, however, measures the efficiency of the system in heating mode, not in cooling mode.
At the end of the day, all I can tell you for certain is that my ducted mini-splits are keeping the house much more comfortable and at a consistent temperature than my 10 year old heat pumps were, and at something around 25% less consumption so I'm happy as a clam. More importantly, wife is happy too.
Are clams happy? Never mind - a topic for another day.