Page impression in overview report

Hello everyone!
I wanted to ask about page impression under overview report in adsense. My question is about total page impression vs channel page impression is not the same, why?
For every blog or web site I created new channel, if you click under top channel it will show page impression according to individual channel and on top it will show you total (???). My question now why total(??) page impression is lesser than channel page impression? for example today channel page impression more than 60k but total only 18K. pls help why.

 

Probably because you have ad blocks with the same channel.

 

In my case, It shows me the average of all the channels in total impressions.

 

Thanks for the answer. Most probably average..I will calculate later.