NM: Replica comparison negative v1hmmsg field contains a value -2 (4313783)

Issue

When I run a replication comparison I see a negative value in the last column, difference between replicas. Why? What is causing this? 

Solution

If the source mail file contains 200 documents and the replica contains 201 documents, then the "Diff" column ('V1MSGDIFF' field) would read -1
if source has 200 and replica has 202 it would be -2

When the replica mailbox has a higher number of messages we see a negative number in the column.
This can be due to the source mailbox contain deleted documents, but if the deletion stubs do not propagate in time to the replica then the replica can have more documents than the source.  This is why you see the value change. After another replication, the deletion stubs clear out the documents on the replica, and both databases contain the same number of documents.
A positive number means there are more documents in the source, than the target replica.