Non Sufficient Funds Notice in mail, Written on it is "Cleared" and a positive balance. What does this mean?

Today I got a NSFN (Non sufficient funds notice) in the mail from my credit union. I've been very good about balancing my account and I'm not sure how I might have overdrawn it. Written on the notice is "Cleared" and "Balance: $2.89". Does this mean there was an error on the Credit Union's part? Or does this just mean the transaction (a withdrawal of $20 on-site) was just cleared? The voice response system says that my account over drawn while the paper record says "cleared". This makes the situation confusing for me.

If it makes any impact, their voice response system has been up and down lately and up until this week their on-site ATM would read my debit card but reject the correct pin number.

Any advice/information would be great, as the office is closed! Thank you!