Why HMS Devastation was not quite a monitor

Could HMS Devastation be considered a monitor because of her low freeboard?

Not under the strict definition; her form approaches a monitor, but her design and intended battleship role keep her outside the category.

HMS Devastation shares the low silhouette and low freeboard associated with monitors. Classification, however, depends on intended service as well as appearance. She was designed and employed as a battleship rather than as the more restricted type implied by a strict monitor definition. The result is a close visual analogy without a true doctrinal match.