It is possible that a Last Mile peer's ISP or local network filters or otherwise alters content that can be observed for a particular web site. It is also possible that a blocking page is presented to the peer, or an ISP is inserting additional content (such as an advertisement) that does not normally exist on the web site being accessed.
You should review your test results to determine the likely source of the unknown hostnames, and to determine if the hostnames originated on a specific set of peers as designated by their machine IDs. Because the additional content is what end users from those locations would normally observe when accessing the web site being tested, the data should be considered accurate. However, to avoid testing from specific peers, you can blacklist them. For details, see Creating a Peer Population Blacklist in the Synthetic Classic documentation.