Why are there orders from the previous month in this billing period?
The way that order attribution works was updated in February 2015. Previously the order attribution was based purely upon the date that the order was placed, or the order creation date.
However, this doesn't account for scenarios such as an order being placed, but not being paid for, or other cases where the order is not actually completed.
The way that attribution works now is based off of the last updated date of the order. So, for accounts that attribute orders based off of the order being in a completed status, if an order was placed on January 20, and not marked as complete until February 5, the order will be attributed in February, not January.