I think the idea is native speakers have practical mastery of the language, with a huge bias towards words that have common use. If its a second language, outside of the words you absolutely need to get by the words you pick up might be more obscure or academic.

For example: Estalogical vs Vestigial. Native speakers will all know vestigial due to biology class, while a second language learner would have no bias on which to learn because neither are particularly relevent. They might pick up Estalogical instead, sound really smart, and no one would realize they never learned vestigial because its not used enough.