Hello,
I believe the warning email is sent 24 hours before your account gets put in the queue for suspension, so if you didn't visit the forum within 24 hours of that email (or whatever the time the email says), then your account gets put in the queue. So, what likely happened was that you saw the email, and visited the forum AFTER that time period, so that your account was already in the queue to get suspended, but just wasn't quite processed yet.
In the future, just make sure to log in at least once every 31 days so that this doesn't happen again.