Should all AD USNs be the same?

I have two AD sites, one with 5 DCs and the other with 2 DCs. I'm seeing most DCs with zero difference between current and original USNs, and only two (on the same site) showing any difference. Is this to be expected?

My thought is that all should see a difference because every DC should see replication and therefore updates should be made on every DC - or do the USNs only change if an update has been made on the respective DC?

There's a good, very long article on Technet that you could probably benefit from reading, titled, How the Active Directory Replication Model Works, but in short , it's exceedingly improbable that any two DCs in a forest will have the same USN at the same time, as the DC USN is incremented each time an Active Directory object is changed on that DC. As such, I seriously doubt you're actually seeing the same USN on a number of DCs, but more likely the originating USN value for the last replicated change, or perhaps the High-watermark (direct up-to-dateness vector) value.

Having said that, the Microsoft documentation specifically states that you shouldn't concern yourself with USNs unless you're having replication issues such as a potential USN rollback. But, if you'd like to anyway, run Repadmin /showutdvec against all your domain controllers and view the local USN reported back from each domain controller.