While I have never heard for certain, my take on it is that with some companies there is the recognition that once your proofs are submitted and your report is accepted, you are due your payment. And you are due your payment unless subsequently fraud is discovered, whether the client likes the report or disbelieves it. Appropriately, your payment is set into the next pay cycle. Companies that do this with minimal variations between them are CoRI, MFI, Service Intelligence, Bestmark and such.
Then there are the companies where you don't get paid until the client pays them for the work. This is so wrong on so many levels, but primarily it is wrong because your contract is with the company, not with the client. It is the company that owes you payment, not some contingent payment arrangement based on if and when the client finally pays his bill. Many have been the discussions on forums in the past 12 months about slow payments, no payments because supposedly the client is/was slow to pay.