It’s funny, but a little worrying, when supermarkets can’t do maths correctly on their signs.
I suppose this is erring on the side of caution, saying you get 20% but giving you 33%, rather than saying you get 33% any only giving you 20%. Which makes sense from a legal perspective, giving you a bigger discount than is advertised.
Although, if a supermarket can’t get the numbers on their specials labels correct, where else are the numbers wrong?