Yes it is correct because many users block monitoring sites like that one.
The information is given by javascript call, that is also mean that users with javascript disabled, or get javascript error on page, also fail to send that info to google.
The block can be done either using antivirus/anti malware programs, ether direct by add some site names on the host file of the system and changing the IP to localhost so they fail to run. Its an easy trick if you try to avoid some sites that monitor you.
If you wondering if there is a way to always give that infos regarding if the user blocks it, yes there is, you send that informations on code behind direct on google server, but its a little complicate.
And one tip - better keep that infos for your only.