CPC
Cost per click
The advertising spend divided by the number of clicks it produced.
CPC = ad spend / clicks
What makes it wrong.
The mechanism, not the warning.
Platforms count clicks differently. Some count any click on the ad unit, including profile and hashtag clicks, and some count only clicks through to the destination. A CPC compared across platforms without checking which click is being counted compares two different denominators.
In detail.
CPC sits between what the auction charges and what the creative earns: it is the product of the cost of reaching people and the rate at which they respond. A CPC can therefore worsen either because the auction got more expensive or because the creative stopped working, and the number alone does not say which.
Splitting it into its two parts is usually the first useful diagnostic step, because the two have different remedies.