True, although if you're in the EU, there should be a 30 days reference period.

"In all EU countries traders are obliged, when offering a discount, to indicate the lowest price applied to the item at least 30 days before the announcement of the price reduction."

Of course, enforcement of such rules is the problem here. But if you notice such discrepancies, you are more than encouraged to file a complaint to the shop and then to your national agency responsible for the protection of consumers if the seller doesn't act in good faith. I was fairly surprised to see how well the latter has worked against a large company in my Balkan country.

Source: https://europa.eu/youreurope/citizens/consumers/unfair-treat...