Exercise 2: Comparing Local DNS Against a Public Resolver — Possible Solution
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WHAT TWO DIFFERENT ANSWERS MEANS
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Per this chapter, "two different answers for the same name is a
genuinely strong signal: the local resolver is holding a stale or
otherwise incorrect answer, while the public internet's view of that
domain is already correct - or, less often, the reverse, if a change
was only made locally and hasn't propagated out yet."

WHY RUNNING BOTH IS MORE USEFUL THAN RUNNING JUST THE FIRST
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Running only the default lookup (against the locally configured
resolver) tells you what answer this particular machine/network is
getting, but nothing about whether that answer is correct. Per this
chapter, comparing it against a public resolver like 8.8.8.8 "usually
tells you within seconds whether the problem is specific to local DNS
infrastructure or a real, global issue with the domain itself" - a
distinction the first query alone cannot make.

THE DIAGNOSTIC VALUE
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If the two match, local DNS infrastructure is very likely not the
problem - whatever's wrong is either not DNS at all, or a genuine,
globally-visible DNS issue. If they don't match, the fault is
specifically local (or specifically not-yet-propagated), which tells
you exactly where to focus next rather than guessing between the two.

WHY THIS WORKS AS AN ANSWER
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It explains what a mismatch specifically indicates per the chapter's
own wording, and explains the added diagnostic value of running the
public-resolver query as a comparison point rather than treating either
query in isolation as sufficient.