I seems that recently Google is resetting the PageRank of domains based on the domain's WhoIs record.
I've noticed that if a domain drops & receives a new creation / expiration date. It's pagerank becomes "0".
I've purchased various PageRank domains & about 2-3 days after catching the drops, the PageRank slips from "3" or "4" to "0".
Also, you may notice services like: DropScout, RegisterCompass & FreshDrop don't display as many PR domains as before. The drop lists now have mixed results for PR, most being fake.
It's a bit confusing also, because there are drop catchers that are catching PR domains from the raw zone files & these do keep their PageRank. It may be low PR, like 2 or 3, but perhaps since these domains aren't publicly displayed as PR domains, may be the reason why they don't lose the PR.