After several bad experiences in chat, I haven't been there in years, but I can offer an email-based possible explanation....

When I check my email at home, don't close the session, then come back and check it at work, it will say I have -1 email, because I've already read the one that was in my inbox, and no new ones have come in.

No clue if that has any applicability to chat, but I tried.

CB