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?