Projected revenue means what revenue is expected to be paid based on the ad requests. The actual revenue is based on the ad impressions and may be different from projected revenue. While the revenue is being paid based on the ad impressions the reporting is based on the ad requests as the ad servers count requests and do not know if the ad request was turned into actual impression.
For example, visitor landed on your site but left before the ads were loaded. The ad tags sent request for the ads to appear, but they didn't, as user closed the page. In this case ad server counted and reported ad request, but advertiser didn't pay for it as it wasn't displayed, didn't become an actual impression.
The same can happen for many technical reasons such as ads placed below the fold, too many ad codes on the page, ad codes from different networks, pop-ups, malware and others.