No where in your post do you reference the global variables the blockchain uses to make these calculations. It's not "interest" but inflation. That inflation is based off the changing rate of VESTs to STEEM. The rules are 15% of the total inflation goes Steem Power. To get the rate of inflation, you use this formula:

current_inflation_rate = (978 - (head_block_number / 250000)) / 100

Are you saying that's not correct?

We get that from 15% of total inflation (our 8.89% number) going to Steem Power holders.

If 15% is the correct amount of total inflation that goes towards Steem Power holders and the total inflation is 8.89%, how could we get any other number but 1.33%?

Am I missing something?

Please, show your work to how you're getting the numbers you're getting using the same global properties the blockchain uses.

EDIT: Yep. I was missing something. :) The "total inflation" can not be thought of as a percentage to be multiplied against. It's more accurately an amount of tokens determined by a formula which uses the virtual supply.