There are, as you say, many pairs of '-ic' and '-ical' adjectives, but no rule as far as I know for a consistent relationship in meanings; some have simply developed in slightly different directions; others are interchangeable. As for your examples: 'tropic' usually has to do with the actual 'tropics', specific geographical regions of the world, where 'tropical' can be used of things that are similar to the conditions there but not necessarily directly related to the tropics per sé (still, in many uses they are synonyms); 'magic' is generally used to refer to the 'real' thing, where 'magical' is used for phenomena that merely resemble it (a magical day, date), though the dictionary lists them a synonymous.
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