ROI and POT are two completely different things.
Return on investment is the return on the capital invested. You allocate $1000 to your betting bank. This is your investment. After a year you end up with $2000 in the bank so you made 100% ROI. Some say they are going to put $1000 in but start with $100. In this case the ROI is 1000%.

POT is profit on turnover. You take the above $1000 and over the course of the year you have 1000 ten dollar bets and end up with $2000 in your bank. In this case you have outlaid $10000 in turnover and made $1000 profit, thus profit divided by turnover gives 10% POT.

You could argue that stake is your investment. Stake (the money at risk) is more an individual investment per venture. Hence while the bank is set up to pursue a specific betting angle, win or lose, only a portion is at risk at any time hence your stake is each individual bet.