Tail aggregate is the sum of all the sites that had less than 200 impressions bid on in a day.

All campaign statistics are synced at 00:00 CET. The data that has come in for the previous 24 hours might not have been consolidated and all databases merged with our inventory sources (depending on their time zones and scheduling hours) which sometime can cause temporary data differences. All differences will be cleared during the next synchronization. When the data is not 100% ready the system will show [tail aggregate] as a site in the campaign statistics.

This is temporary and will disappear in the next server synchronization.