AR Status Date vs Accounting Date, when should they be different?

We noticed our GL Outstanding Balances did not match up with Open Invoices available that were open in AR for a given vendor. Since I wrote these reports in SQL, I wondered if I'd made a mistake. When I looked at the table data to see if my reports were off, I noticed that the Status Date and Accounting Date for two transactions were the same date but had different years. This accounted for the variance.

I've tried searching for more info on the status date to understand why these fields are different but am not finding much. I don't think for our use, there should ever be a difference between these two fields and I'm not sure how it happened. What is the difference between the AR status date and accounting date and how are each changed? 

Thanks!