What do you mean when you say that "generalizations are false"?

I have only come across this here on fluther, and have never heard an explanation.

Do we not – out of necessity – speak in generalizations all the time? When we say that humans have two legs, we’re in no way saying that someone who lost both legs is not human. When we say that people are social creatures, that isn’t to deny the humanity of those who prefer to live the life of a recluse.

Anyway, I don’t want to put words in anyone’s mouth. Maybe you could describe what it is about generalizations that make them “false”.