The ROI is based on the intial amount invested. PPT is held as collateral and never sold unless you choose to sell your PPT. PPT is used to access the liquidity pool (see my other posts) to funds the purchase of invoices. When the invoice settles the PPT is return back to you + the interest. Once PPT returned to you, you can re-use the PPT again to access the Liquidity Pool to buy more invoices. Repeat, wash and rinse.
Have you heard about the "Secret Sauce"? When PPT is $10, you have $8 purchasing power. When PPT is $100 you have $80 purchasing power. You take advantage of price appreciation of PPT when buying invoices.
If you use fiat to purchase invoice worth $4545, you need $4545 and return is 5%, the ROI is also 5%. If you purchased PPT at very low price at say $0.33 and the value of PPT goes up to $10 (say, in the example) then you have a purchasing power equivalent of $4545 for initial investment of $1500. Hence return of 5% but ROI of 121%.