For some reason setting the post_type query variable to “page” doesn’t have the desired effect. Other post types (“post”, custom post types) work, but “page” doesn’t do anything.
This is not a Relevanssi bug, but a WordPress feature (see this trac ticket). If the user inputs a bad value to the post_type query variable, WordPress changes it to “any”. For some reason that happens also when the post_type is set to “page”, even though that should be a valid value.
There’s nothing Relevanssi can do about this. However, there’s a fix. Relevanssi introduces a new query variable post_types, which happily accepts “page”. Use that, if you need to filter by post type.