A discount is only as meaningful as the original price it is measured against. When a retailer lists an item as reduced from a higher price, that reference figure is doing most of the work in making the deal look attractive, which makes it worth understanding how that number is set in the first place.
In most markets, retailers are given fairly wide discretion in choosing the reference price used to calculate a percentage-off discount. In some cases it reflects a price the item was genuinely sold at recently. In others, it reflects a manufacturer's suggested price that few, if any, retailers ever actually charged, or a price that was set briefly and never intended to be a common selling price at all. Both practices can produce the same appearance on a price tag, even though only one represents a real discount from prior selling behavior.
A few habits make it possible to judge whether a listed discount reflects a genuine reduction.
None of this means every advertised discount is misleading. Many reflect genuine reductions from prices the item was recently and commonly sold at. The point is that the percentage-off figure alone is not a reliable way to judge value without some sense of what the item has actually sold for over time.
The more reliable question is usually not how large the advertised discount is, but whether the final price is a reasonable one for the item, based on what it has cost in the past and what it costs elsewhere. A modest discount from an honest reference price is generally a better indicator of value than a large discount from an inflated one, even though the second option often looks more dramatic on a price tag.