I'll see if I can get a direct answer to this. The goal, as I recall, was to make it impossible for two disks to ever have the same PVID. By having the machine serial number be one part, and LVM serialized enough such that only one disk will be given a PVID, and that it is expected to take at least one second, no disk PVID should ever be the same.
Once the PVID is made, it does not matter where or when it was made - only that it has the correct length and is unique in both the system and the volume groups (if any) it is used in.
Just guessing - but the time() might be used as a seed to random() - which could explain why it is not as simple as the delta from epoch.