> "On sale" usually means "the price is the same it's been for the last six months, but we're showing a big discount on a price that was never charged", in my experience.
In many/most jurisdicitions, this isn't permitted by regulators (e.g. it would typically qualify as false advertisement), but enforcement is mixed. You can help regulators by reporting it and including supporting documentation when you see it.
Add a few dozen items to an Amazon wishlist a few weeks before Prime day. Take a screenshot. Come back on Prime day and see how many are "Prime Day Deals!" with roughly the same price they used to be.
In many/most jurisdicitions, this isn't permitted by regulators (e.g. it would typically qualify as false advertisement), but enforcement is mixed. You can help regulators by reporting it and including supporting documentation when you see it.