There is an inverse relationship between maximizing ROI and generating maximum sales.

Here is the most obvious example of how that works. You can generally increase overall revenue and the quantity of sales by increasing spending, especially on general keywords. Doing this will reduce ROI because advertising costs as a percentage of sales must go up to do this. There is a definite point where your income will be highest but not profitable.

You can increase ROI by reducing spending, especially on general keywords. That is very likely to also reduce the number of sales you generate and your overall revenue. The sales you do make will be more profitable due to the lower cost of advertising - there just won't be as many of them.

So the challenge is to find a balance between maximum revenue and maximum ROI per sale. If you have a high markup and can afford it you can skew your advertising more towards higher revenue and lower ROI per sale. If you have a tight budget or low profit markup you will want to slide your spending more towards maximum ROI.

The first can increase the speed of growth while the second can generate gradual increases in growth with lower risk.