Frankly, I don't know, either.
From what I learned in this thread, VMware Tools seems to slowly advance the clock when the guest is behind the host. But apparently, this isn't true the other way around:
Just to make sure: To be able to reproduce this issue, it's important to know that network time server sync is disabled in the guest, of course (see my first message where I reported this issue). It wouldn't make sense to enable the Sync Guest Time With Host option if that option would be enabled.