Why does the English language allow one to select between, say, biologic and biological, neurologic and neurological, and technologic and technological? Why complicate our language lives with the choice? Is the universe malicious?
According to one study, the suffix -ic is preferred over the variant -ical by a ratio of 8 to 1. Curiously, however, when -log precedes the suffix, the ratio is reversed. (In another example of this phenomenon, called potentiation, -ness is much more common than -ity — except when the suffix is preceded by –able.) But that doesn’t answer my questions.
For the most part, the choice seems to be personal or institutional preference, because there’s usually no distinction — no logic, for example, to selection of -logic or -logical. For example, the style guide of the American Academy of Neurology prefers the shorter form, but in other contexts, neurological prevails.
One researcher points out that, as you might have guessed, -ic (from the Greek suffix -ikos) was the original suffix; -ical, formed by adding the French suffix -al, came later. For the most part, usage organically caused a divergence, so that, for example, a historic occasion is memorable, whereas a historical occasion is one that merely occurred.
For another example, economic refers to economics, while economical is used more generally to refer to the quality of economy. In this case, as with some others, the former can mean the same thing as the latter but seldom does. Comic and comical, and geometric and geometrical, are two of the many other sister terms with both (occasionally) identical and (usually) distinctly different meanings.
Sometimes, one form predominates for obvious reasons (fanatical, for example, developed in favor of fanatic because the original form came to be applied as a noun), but in other cases, the variation — for reasons seldom clear — triumphs (botanical versus botanic, for example.)
So, which form should you use in a given context? The dictionary is helpful for most -ic/-ical debates, but the -logic/-logical (and -logous!) issue is an outlier. In such cases, consult an authoritative source.