Agent Total Response Time field discrepancy: exact definition please?

Summary

Unclear on the discrepancy between this value, and the datediff from session creation to agent engagement

Content

In building some basic chat reporting, I want to report (among other things) the time from when a customer requests a chat session, until an agent actually engages on the Chat.

I thought "Agent Total Response Time" (chats.agent_resp_secs) would provide this value, but as a check, I built a separate column "date_diff(chats.first_engaged, chats.requested), and found the numbers differ significantly on every incident.

I suspect I am misunderstanding the exact definition of this field.  Can someone clarify?    Thanks in advance!