I agree with Neil. Both uSNChanged and HighestCommittedUSN are not replicated so a different value is retrieved from every DC. When I tested I found that the values on any DC are close, but not identical. Of course the values constantly change with each replication cycle, but it seems to me that HighestCommittedUSN is always slightly larger that the largest uSNChanged value on the DC. I suspect that when I filter with (objectCategory=*) I only see objects in the domain partition and I am ignoring objects in other partitions (like the configuration and schema containers). When I get a chance, I may check this.
Richard Mueller - MVP Directory Services
