This has been explained many times. It is called adaptive replication. On some of the smaller items (I believe they are PPSE, PPS, SGS and TPR Sieve) have the ability to be validated on a known good host, if the workunit is not a Prime, or in the case of TPR Sieve (which doesn't specifically look for primes) has been consistently sending good work. Not all units are this way, only a small portion of them are. The smaller numbers fall off the top 5000 pretty fast and it's a way to try to make sure none of them go below that threshold. If a unit is declared a prime, it is always double checked in some form or another (and many times tripled checked via backend checking).
So, if you machine does a lot of clean work and it is deemed stable, you may see a few of these units come across your plate. The project does not give out the backend information on what this process entails to deter cheating. Also if a residue comes back that seems wrong, it will get double checked anyway.
So if you are getting these, be thankful that you do knowing that PG systems think your system is worthy of doing this work.