Not every advertised discount reflects a genuine price reduction - a well-known retail tactic involves briefly raising a price before a sale, then "discounting" back down to roughly where it started.
A price-tracking tool showing the item's actual price over recent months reveals whether a "40% off" claim represents a real drop or simply a return to a price the item was already sold at recently.
A "sale price" that's still higher than another retailer's regular price isn't really a good deal, regardless of how large the advertised discount looks - checking a second retailer catches this quickly.
A discount that's been running continuously for months under a "limited time" label suggests the sale price is effectively the real price - worth treating it as the baseline rather than waiting for it to somehow drop further.