Sometimes ads are shown but you get nothing for them, so yes, what you say is correct.
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I find this happens mostly in the mornings. You can't control when users use your app and the time when advertisers choose to pay. Advertisers want people to see their ad - if only a few people see the ad, they dont pay. Another explanation could be that they only pay when the number reaches a certain threshold. Admob dont tell you this kind of information. I've wondered about pretending to be an advertiser and seeing what Admob offer me. Also, look at Infinito for example - whats 6/1000th of $1.00 eCPM? Assuming it was $1.00 at the time. The only thing consistent with Admob is inconsistency.
Also, look at this:
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eCPM is erratic. $5.88 then $0.13 on Xmas Day. I averaged $1.19 across all of my apps over the last 28 days. Great eh?