DNS question: Avoiding circular dependencies without glue records?

Can someone help me the understand the following:

When the authoritative name server for a domain (e.g. ns1.example.com) lies within the domain over which it has authority (e.g. example.com), a query (e.g. for example.com) to the parent domain (e.g. .com) will include both the NS RRs, to delegate authority of the domain to the nameservers, in the answer section and the corresponding A RRs in the additional section, this is know as glue records. These glue records are essential to avoid a circular dependency, yet the Netalyzr study found that only 61% of resolvers accept glue records when the glue records refer to authoritative nameservers. How do the other 39% of resolvers actually work then, given that its very common for the authoritative name server for a domain to lie within the domain over which it has authority ?

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