I am wondering what the kagami biraki promotions to ranks under 5th dan mean. Is it typical for hombu to reserve announcing any ranks given within the dojo itself until kagami biraki? I imagine an unfortunate number are posthumous. Are some strictly honorary?
All of the ranks that they list at Kagami Biraki are by recommendation - those ranks only come on Kagami Biraki. That's why there are fewer ranks listed below 5th dan since most of those ranks come through testing - it's cheaper (about half the price).
As a mental exercise
try calculating the fees that come into hombu from the Kagami Biraki promotions alone - remember that 7th dan can go for as much as $5000. No guesses on 8th