Why do vocabulary sets used by one person within the same context differ as a result of the environment of execution? For example: reading, writing, speaking, listening, etc.
What are the "environments of execution" of significance that are observable?
Are there any constants in the usage clustering? For example, all spoken words are a subset of written words, but words read are not a subset of words written. I don't believe this to be the case, just giving an example.
EDIT: Limiting the scope of the question to only English is fine.