In the end, a sales estimator is just that, an estimation. No estimator is going to be perfect, and so we must take the information available to us to make the most educated decision.

If AMZScout and JungleScout are showing significantly different numbers, then you may want to use the Manual Review Method in order to get a baseline. Let's say AMZScout is showing 500 sales a month, and JS is showing only 150. If the customer review method estimates about 90 sales a month, then I would assume it is closer to 150 and not 500.

Another option is to just take the more conservative number to stay on the safe side to ensure you don't overestimate your sales.