Yeah, all the work is done at the outset in designing the different tests that will be available, not after the tests have been taken. That's all explained in the AICPA documentation on the website.
The explanation that Pikachu posted makes sense. Here's how I interpret it based on my research:
They have several versions of the test in that window, and each test has its own answer key. They have to run each test against the correct answer key, and their process of making sure they're using the correct answer key for each version is to run a whole bunch of them and then look at the passing scores that are coming out as a result of that. If the scores are coming out in the expected spread, then they can assume they used the correct answer keys. If something is off, then they have to check and see what happened. They need a large enough amount of data in order to do all of this or it won't be meaningful. So that's why wave 1 is long and also takes longer to get the results.
AUD 84 (1/9/14-Wiley books/TB + free materials)
FAR 83 (5/21/14-the above + NINJA 10 Pt Combo Lite)
REG 84 (7/9/14-Wiley books/TB + NINJA Audio/FC/Notes)
BEC 76 (10/5/14-Wiley books/TB + NINJA Audio/FC)
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