The "norm" is the baseline average.

Why do you think X is inappropriate in a context?

Because you believe the other people will think it's inappropriate.

Not what you believe, what you believe others believe.

You know your internal mind but not others' internal mind.

So this emergent belief of what other people believe is more stable and takes longer to diffuse than if you based it on your own beliefs.

Because it can take you longer to notice they changed their mind since you can only see external signals of it.