I'm assuming you're using agent groups to pull the report? If so, if the agents in that group change from the time the data is collected to the time you run the report the numbers are going to be different.
The agent groups don't write to a table for their specific data during the time frame it's collected. An agent group only defines who should be in the group when you run the report. So if you have John, Mary and Jane in the group for the month of February and they are all in skill 1, if you take Mary out of the group because she leaves in March and then run the report in March, all of Mary's stats are not going to be taken into account. Visa Versa if you add people.
- Stinney
“The man who asks a question is a fool for a minute, the man who does not ask is a fool for life.” - Confucius