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Campaigns being upgraded/downgraded during the day so that sometimes a different ad will show up.
Each skyscraper has 4 ads, there could be a big difference between how much ad #1 pays to what ad #4 pays. Same thing for only the two ads in the banner.
The ads are targeted to the page first and to a lesser degree to the theme of the site. Not all pages get the same ads.
Inevitably you will get your share of these.
In addition, times like weekends can get lower bid adverts published, if no others are around