Custom 404 page always seems to be cached?

When I update the page which is defined as the Default 404 (not found) page on admin/settings/error-reporting, the page is instantly updated (as I would expect) when I view it, but not if I generate a 404 error (the old version is shown). This happens even when I am logged in and when page cacheing is disabled.

What seems to be happening is that something (the default 404 page) is being stored in the cache_menu table?

Is this by design, or a bug?

Cheers
gpk