What does it mean when impressions disappear from AdMob statistics?

Within my game I preload interstitials and then may display them later (if the user stays in the game long enough). As such its not unusual for requests to be larger than impressions. But recently I have noticed something strange happen:

[ol]
[li]Requests go up
[/li][li]A little later impressions goes up
[/li][li]Then 5 or 10 minutes later the impressions goes back down.
[/li][/ol]

What does this mean? Why does this happen? Where these intersitials shown to users or not?

I would ask AdMob themselves but we all know where that would get me