I saw too many mistakes about these 2 things. Many websites and forums are showing the wrong answer. That's because Google Adsense first introduced the RPM and then replace this single value by 2 new datas : "Page RPM" and "Impression RPM"
"Page RPM" and "Impression RPM" are 2 different things. Simple way to prove it, a screenshot from my Adsense account :
You can see Page RPM and Impression RPM are different.
So, here is the explanation :
Page RPM = Revenues / Page Views * 1000
Impression RPM = Revenues / Ads Impressions * 1000
1- Impression RPM = classic CPM
If you want to compare your CPM from another ad network with your Adsense ads : check "Impression RPM"
2- Page RPM is useful to understand your website performances
If you want to monitor how much money your website is making on a page base : Monitor and compare "Page RPM". That's a great KPI if you want to optimize your revenues : editing your ad positions, or adding/removing new ads in your templates.
Source : -Quora
This answer is from above source and looks good