The number of impressions being recorded for advertisers, campaigns and banners and/or websites and zones are significantly lower than the number of requests.
As soon as Revive Adserver has determined which banner will be displayed, this is logged as a request.
When using an Image Tag, No Cookie Image Tag or XML-RPC Tag, Revive Adserver will also log the impression at this point, as there is no other way for these tags to record impressions.
However, when using any other kind of Zone Tag, or when using the website-level Single Page Call Tag, then Revive Adserver uses a "beacon" to record the impression - that is, after the banner has been loaded, an invisible element is loaded in the page which calls back to Revive Adserver to let it know that the banner has been successfully loaded in the web browser.
This is done to provide a more accurate record of the number of banners that are actually loaded (i.e. impressions) vs. the number of requests to load banners (i.e. requests).
If you are using Single Page Call Tags, review your use of the tags.