That's the official inflation numbers, which are lies. It's an open secret that in the 90s there was a political movement among the owners to artificially lower reported inflation (see: Boskin Commission for example) because it would boost the economy (on paper) while letting them absorb the gains.

Underreporting by 1-2% a year doesn't seem like much until you do it for several decades. Looking at actual inflation instead of the reported lie, that mars bar is closer to $2 today. Shrinkflation had to happen to keep the bigger con going. Most businesses will, of course, cheat the margins at every instance.

TLDR; shrinkflation is a symptom, not the disease