NS addresses do not match list from parent zone.
Can anybody explain what this message means or point me to a document that explains further?
Generally what this is referring to are consistencies, or lack thereof, between the delegation of the child zone (subdomain) and parent zone (domain). When a subdomain is delegated from a domain, the domain zone specifies the authoritative name servers for that subdomain. This delegation also occurs with first-level domains, e.g. domain.com -- the .com nameservers delegate domain.com and identify the authoritative nameservers.
Then, in the actual zone file on the authoritative nameservers, there are also NS records specifying the nameservers for that actual domain or subdomain. They should match the actual delegation. What often happens here is that the original domain registration (or subdomain delegation) is created with a couple of nameservers, and then sometime later more nameservers are added, or the nameservers are changed.
In short, you'll want to check the list of nameservers provided in the report against the original delegation. Presumably they're not present.