hmmm...right now i can't think of any rule. some adjectives end in "ic" (eg tragic), others in "ical" (eg hysterical). in some cases, both "ic" and "ical" forms exisist (eg ironic and ironical) with both having the same meaning. however, i THINK ironic is the more commonly used form.
for economic/economical, classic/classical and probably lots of other pairs of ic/ical words , the meaning does change depending on which ending is used. is there a rule on which ending to use? i still can't think of one! i guess it's just one of those things that just are that way because they are that way