maybe an analogy?
spellcraft is like you knowing the fine-tunings of your car's engine, and could probably rebuild the engine block from the gears up should something go awry. you can identify problems based on description alone and can cite the product code and year of production, as well as other alternative part makes by the same company that would also work.
knowledge (arcana) is you know where the parts for the engine block are made and the names of the original designers of your engine, as well as where they studied earlier and got their inspiration. you know popular opinion concerning how well the engine block performed based on lab tests and have studied up quite accurately on benchmarks comparing it to other makes. you could probably change your oil, but you're more of a scholar of engines.
?