§142

The rules for the use of words prescribe their use in normal circumstances. If circumstances were to change dramatically then we might well be at a loss about what to say. The disappearing/reappearing chair (§80) is an example of this kind of thing (is it correct to call it a chair? - We would be at a loss about what to say.)

Similarly, if people in pain didn't typically say "my ___ hurts!" or wince or say "arrgh!" but behaved differently half the time (perhaps they might say "Mmmm. I'm in pain" and look completely serene/contented/smile) - then we'd be unsure about whether the word 'pain' was being used correctly.

Presumably a similar point could be made about understanding. - If someone went on to use an expression in accordance with the rules for its correct use for the most part but used it differently on some occasions we might be unsure about whether to say that they understood it.