It does not add up because, as is typical with forums and blogs, you are only seeing part of the story. Go back to the example I posted with the fixed fee of $650. What the AMC grosses depends on what the appraiser charges, and there are times when the appraiser charges more than $650, and the gross to the AMC is a gross loss. The AMCs accept that because the agreed upon fixed price is designed to produce an average gross of $X. Appraisers are not posting examples where the appraisal fee exceeded the total fee paid to the AMC, but that happens more often than most appraiser would believe.
As you note, if an AMC made an excessive gross on every order, they would not stay in business - because the lender would just seek a lower cost option. The reality is that on most deals they make around $150-$200, on some they make more, and on some they make less, or even take a gross loss, because the losses will be offset by other orders where they make more.
By the way, most appraisers operate in a similar fashion. Most have a set fee that covers 80% to 90% of assignments. Some of those take longer than others, so what we make on each assignment is highly variable. And, sometimes we do something at a lower fee than it really deserves, because the client is a good one that provides us a lot of work.