Correct me if I'm wrong, but I don't think the APR is based on SBD/Steem price either. Wouldn't it be based on how much people used the bots and then the bots disbursement rate of that received amount? Price fluctuations would indeed change the voting value of the bots, but afaik affect all bots proportionally to their SP.
So say if people decided to boycott one of the voting bots and their activity dropped to 0 then the APR would also drop to 0.
Likewise it's true that it's not predictable, only helpful as a benchmark of past results. With delegation you're picking your pony and so far tipu has been a steady bet.