Power/HR vs Efficiency

According to the definitions shown in the page, power/hr is:

Also known as output / input ratio, a measure of how much output (watts) you produce for a given input (heart rate measured in beats/minute).

and efficiency is:

Efficiency Factor is normalized watts (output) divided by average heart rate (input)

They look pretty similar to me, but I am sure they measure different things (otherwise they wouldn’t be shown together in the activity metrics). What is the difference? Which one should I care more to tell that I am getting aerobically fitter?

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Power/HR is Avg Power/Avg HR
Efficiency is Normalised Power/Avg HR

I have wondered about this myself and don’t know the answer. There is a linear relationship between power and HR (until threshold anyway) so I am not sure why “Efficiency” uses normalised power which will break this linear relationship.