Mystery Invoices

I must be missing something.  I was looking through USAC's 2013 Annual Report, and I came upon the chart on page 15, showing invoice processing, and I noticed the numbers didn't add up.  So I dumped the data into Excel and added a row at the bottom:

	Q1	Q2	Q3	Q4
invoices received	196,428	333,079	451,211	520,444
invoices paid	176,272	302,112	405,089	471,081
invoices rejected	10,740	18,874	24,865	30,813
???	9,416	12,093	21,257	18,550

Let's look at Q4 as an example: USAC got 520,444 invoices, paid 471,081 and rejected 30,813.  What happened to the other 18,550?  They can't be dumping thousands of invoices into a black hole, could they?  Maybe the dreaded "pass zero" invoices are in that row; those invoices wouldn't fit in the "rejected" row, since they are technically approved, but they wouldn't fit in the "paid" row, either, since they are approved for $0, so there's no payment.

Anyone out there have a guess?  Or maybe someone could give us the actual answer....