This is a minor annoyance and in no way represents a problem.
I've noticed on a couple of WordPress installs I look after, that if the host has mod_suphp installed (presumably similar too) WP Super Cache will report that wp-content and public_html are writeable.
mod_suphp makes PHP run under the username of the account executing the PHP code, which means anything chmod'd 755 will natively be writeable due to the ownership permissions.
Maybe for a future release, a way of detecting if mod_suphp (or similar) is installed and disable the wp-content and public_html permissions checks.
Keep up the good work!