I'll make this super simple for the standard person that wasn't amazing at math.
Creating the numbers and gains out of air.

You start with 10 sp, at 10% interest a month you have 11 sp.
You start with 11 sp, at 10% interest a month you have 12.1 sp.
You start with 12.1 sp, at 10% interest a month you have 13.31 sp.

Now over time steem may go up in value or may crash and burn and become the value of rdd, diem etc.
That being said today's price does not represent a price seen 104 weeks from now. So while 10 steem may be worth .05 btc currently, your 50 steem in 2 years might be worth .05 or .000005 btc.

https://steemit.com/steemit/@rogue91/steem-power-and-proper-calculations-of-gains