Projected revenue means what revenue it is expected based on the ad requests. The actual revenue is based on the ad impressions. While we 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 if the site is slow it will take an extra time for the ad to load, therefore if user is going to leave the site or browse to another page the ad won't load, therefore won't be paid, but the ad request would be sent and counted by the reporting. 

The same can happen for many technical reasons such as: user browsing site too fast, ads placed below the fold, too many ad codes on the page, ad codes from different networks and others.