With the last HF we went from a small to a much bigger reward pool. But it took a while to fill this pool up. Basically an unforeseen consequence....oops :) That's why the payouts were low for a while. Not enough Steem in the pond. It didn't bother me I just kept writing.
A hard fork 'forks' the Steem blockchain into the original chain (hf18) and hf19. If consensus is reached everyone follows HF19 and this will become the new chain. HF19 has new rules. (linear reward curve) A next hard fork will 'fork' HF19 in to the original chain (hf19) and HF 20. And it goes on and on like that.
Sometimes ppl don't like the new chain and they stay on the old one. this is how Ethereum Classic was born.
Very analythical @exyle ....thanks for the update.