Top N truncation
The Search Console API returning only the highest-volume rows for a query, so the returned rows never sum to the site total.
What makes it wrong.
The mechanism, not the warning.
Summing the returned rows and presenting the result as the site total is the single most common Search Console error, and it always understates. The site total has to come from an unsegmented query, not from adding up a breakdown.
In detail.
A request returns rows ordered by clicks up to a row limit. Everything below the cut is absent from the response, and the response says nothing about how much was left out.
For a site with a long tail, which is most sites, the withheld rows can be a large share of total traffic.
Through an agent.
What changes when a model asks instead of a person.
A model handed a list of rows will add them up, because that is what a list of numbers invites. Only the tool description saying the rows are a truncated top N prevents it.
Where this is measured.
Top N truncation comes from Search Console. The server page lists every tool that returns it, with its parameters.