Chat Total Discrepancy

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I have 2 columns, each returning different chat totals and I'm not sure why.  My columns are:

• COUNT(chat_agent_sessions.chats_completed)
• SUM(IF(date_diff(chats.first_engaged, chats.requested) >= 0,1,0))

When I run these columns individually using the same filter (chats.requested), my totals are different.  We're a 9-5 company so in theory when I run a daily total, I'd expect the totals to match.  My questions are:

1. What defines a session for the chat_agent_session table?
2. Could this have to do with the join between the chats and chat_agent_session tables?  I'm thinking if an agent has multiple chat sessions in one day, this may be causing duplicate rows.