Part of the reason people are misunderstanding you is that terminology got abused a little bit upthread.
"I just always assumed that all the variables present in a closure are maintained" should really be (something like) "I just always assumed that all the variables present in a scope that produced a closure are maintained". You're right that a closure only needs to contain the variables that are closed over, and that some of those variables are scoped such that they can appear in multiple closures.
Sure, but that's the only exception, right? If there's no eval call in the function that requires the closure it's easily determined what variables don't need to be included, right?
"I just always assumed that all the variables present in a closure are maintained" should really be (something like) "I just always assumed that all the variables present in a scope that produced a closure are maintained". You're right that a closure only needs to contain the variables that are closed over, and that some of those variables are scoped such that they can appear in multiple closures.