It's best to explain the mechanism behind Steem Power. It's really unclear if you don't know how it exactly works.
SP is actually VESTS. A vest is like a share of a pool of money. When you power up, you buy those shares. You don't get any interest in a sense that you'll get more vests.
Instead the value of those vests will go up because the pool of money will grow bigger over time. total_vesting_fund_steemshows how much steem is on that pool of money. total_vesting_sharesshows how many shares exist.
I think it might been a little mistake to talk about Steem Power as steems and not as vests. Of course it is easier for average user when they don't have to know about vests, but it makes the system harder to understand if somebody really wants to.