You have a point, and in truth, there is of course no hard dividing line. But I think there
is a line of sorts.
One example: Poundland pack of Kit-Kat, "50% extra FREE!" - 9 bars instead of 6 for your quid. Fine. Then Poundland decide to reduce the pack size to 8. So what do they do? Reduce the 'FREE!' flash? No. They decide between them to treat 5 bars for a quid as 'the norm', and bring out a new 8-pack bearing a big flash: "60% extra FREE!"
Now I don't think Tesco, ASDA, Sainsbury,
Morrisons or even Waitrose would behave quite that way. It doesn't surprise me that Poundland would.
But it does surprise me that Nestle would get together with them in an office somewhere and agree to 'do their dirty work for them' by producing the new special, unique and deliberately misleading packaging.