Both tools give you a range: low, typical and high. No outside estimate can be exact, and the range shows how far the real number can sit from the typical one.
Sales Estimator looks at how many players reviewed a game and how games like it usually sell for every review. You get estimated copies and revenue, what the studio keeps after Steam's share, and how the closest similar games sold. A game needs a few reviews first.
Wishlist Estimator turns a game's Steam followers into a wishlist range, plus how many it gains a week, next to similar upcoming games.
They are good for comparing games and spotting momentum, not for exact counts.
Tip: want your real numbers? Connect Steamworks on a paid plan, and your rivals get estimated using your own sales.