I don't understand the math. I put in 2000 monthly rent, initial investment 50000, and 36 months of stay. That spits out a return of 72784, so my capital grew by 22784 dollars. In other words, in this particular example, 632.89 dollars per month were saved up for me. How is this 7% of something? What's the formula for calculating this?

I first thought the system might be crazy and promise 7% monthly compound interest since 2000 * 1.07 ^ 36 = 22847.88, which squares pretty well with the number above. But doubling the time to 72 months only grows to a "profit" of 63000 dollars rather than 261000. So that's good, but then I don't understand how this is calculated.