As I said, there's no "One" answer. It's usually a combination of factors that give the end result.
Just using my 3 channels as an example.
Pinball/Classic Arcade - videos average 2 minutes in length. 90% male audience from USA/Canada/Australia/France - RPM is what I'd consider low (people on this forum consider it average)
Healthy cooking channel - Videos between 4 and 7 minutes in length. 50/50 male and female audience from USA/Canada/UK - RPM is 3x that of pinball channel.
Fitness channel - videos between 30 and 60 minutes in length. 60/40 male/female audience from USA/Canada/UK/Europe - RPM is 5x that of pinball channel.
So with the above examples, is it the video length? the subject matter? I don't know