This is true ONLY if the hodler of the 17000 SP doesn't power down at all, from the moment you power down your earnings per day become less and less after each week, because you have less SP now, do you really expect someone who has 16840 SP to earn the same "interest" as someone who still has 17000 SP?

Also, as I already told you, each subsequent power down, powers down MORE SP than the previous one. I tried it and I know it. I suppose some earned interest is added to your power down amount as the time passes.