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Exchange B2D Byte Count different than duplicate job

Level 5
Since upgrading from 11D to 12.5, we are backing up to disk first, then Duplicating the B2D file to tape.  For every job except the Exchange job, the Byte Count between the B2D and Duplicate jobs are the same.  However, the Exchange jobs show different Byte Counts.  Every exchange backup is a full backup with GRT, and we have been able to granularly restore a public exchange folder from backup.

This table shows the last 3 backups:
When job ran   B2D Byte Count       Duplicate to tape Byte Count
Tuesday night  84,638,591,670               84,643,834,648
Monday night   84,870,426,124               84,875,669,102
Weekend        84,854,696,462               84,859,939,462

Why doesn't the byte count match?  It matches on all 8 other backup jobs!

Level 5
Yesterday I disabled compression on the Exchange backup-to-disk job (it had been hardware compression, else none).  I figured this would get the Byte Count to match, but this morning the number of bytes duplicated to tape were more than what was created on disk by the Backup-to-disk job.

How can the duplicate job copy more bytes to tape than exist on disk?  Does this make sense to anyone, or does no one else have this problem?  Is there no explanation?

Level 5
I guess the exchange duplicate job's byte count is always exacly the same as the exchange backup-to-disk job's byte count for everyone else?