VMware block level differential performing "full" size backup

I have noticed recently whilst examining slow SLP processing that some of my weekend VMware block level differential backups, although showing a low file number count are showing a "full" sized backup as having been performed.

My VMware backup policies are iSCSI (SAN) or NBD based.  They auto select VM's based on VM Version; vmx-04 VMs have the "perform block level incremental backups" box un-ticked, vmx-04 polcies have this box ticked. 

In my case I do a full backup once a month on a given Sunday and differential backups on all other Sundays.

When I do a report of a random number of VM's backed up and their histories I see full and differential backups where the file count value is as expected for differentials BUT the file size is essentially the same as the full backup.

Althought mostly Windows VM's affected I also see some Linux based VM's affected as well.

NBU Version across the board is v7.1.0.4.  vSphere version is v4.1.0; ESX versions seem to be a mix of ESXi v4.1.0 running various patch levels and as I mentioned above; happening for a couple of policies which use both NBD and SAN based backups.  Policies backup to MSDP units.

Can anyone shed any light here? 

I have performed new full backups but the issue seems to return after a number of weeks (i.e. before the next full).

Thanks,

Malcolm