If some Automatic Call Distributor (ACD) calls are transferred, the result can be that two or more call legs fall in different hours of the day. Therefore, the call is counted once in the first report and twice in the second report.
The software is working as designed. If this is not acceptable, you could alternatively count call legs, instead of entire calls and the totals on the first report and the second report will match. Then select count distinct sessionID becomes select count distinct (sessionID, sessionSeqNum) pairs. However, this would change the definition of counting calls for the entire call center and is not recommended.
For more information about resolving Cisco Customer Response Solutions (CRS) issues, refer to these documents: