Best way to have complete review and answer is by logging a support case as usually it is a bit too complex to have it addressed in the community. With that being said, please provide more info about your setup:
Are all VCs metered if they are more than 1?
Support may also request support bundle/DB dump to validate the result.
I ended up talking to VMware support who said there was a bug in 3.5.1 and told me to upgrade and then compare the 2 UM stats over the next few months
4 VCs metered