Downstream server has less updates than the Upstream server.

Hi, I have a DSS (Downstream server) that syncs the metadata from a USS(Upstream server) at our place. I counted the number of updates in the database using the `IsLatestRevision` field in the revision table. They are different on both the WSUS servers.

The DSS has less updates than the USS. The query is as follows. 

SELECT COUNT(*)
  FROM [SUSDB].[dbo].[tbRevision]
  where IsLatestRevision=1

Is this the proper way to count the number of updates, if yes, why there is a mismatch. 
Please help, i am confused on this. Any help appreciated.