Helpful post from the free dictionary on the difference between -ic and -ical as suffixes. It says “One possible reason why the ‘-ical’ variant has arisen in the evolution of English (and caused such confusion between the two suffixes) is to create adjectival forms of nouns that naturally end in ‘-ic.’ In such instances, we actually add a different suffix, ‘-al,’ to the end of the noun, which in turn creates the ‘-ical’ ending.” The entry also provides a list of instances where the different endings indicate markedly different meanings (e.g., politic, political.)