If some item x is definitely f (i.e. it definitely has the property of f-ness), then it must be f in some perfect, absolute, non-finite way.
By contrast, if some item x is definately f then x is f by definition, or is the very definition of f-ness.
The former is an ontological claim, whereas the latter is conceptual/semantic.
That's why all us clever people who spell by rational deduction rather than blind repetition use both spellings, to distinguish our precise meanings.