Invoice Pending after Payment?

I have a store item that can be bought by guests. I'm sure I don't fully understand the workflow so I'm mainly curious how or why an invoice is still pending and "waiting for payment" after the payment transaction is approved?  

There is a linked transaction showing the payment received and approved.  I'm fairly sure in the past in these situations the purchaser has an account created when the transaction completes, but the email address (and real name, and several known aliases) associated with this purchaser are not listed in members. The 'validating' filter shows nothing  and I have 'require email validation' OFF for store purchases. 

I think I'm OK since I can associate the event attendee to the PayPal and invoice information, but this is *really* going to confuse our treasurer. Also me