On the other hand. there is a significant distinction in the uses of historic/historical. There is some overlap, but "historic" is used for important events in history, while historical is used for anything in history, important or not. Hence, a historical period is any period in the past, but a historic period is one that has some special significance.
On a language site, such as this one, we use the term "grammatical". It has two meanings: of or related to grammar and conforming to the rules of grammar. There is another form of the adjective "grammatic", which has the same meaning. I use the words differently. I use "grammatic" for the first meaning and "grammatical" for the second. I'm not sure that everyone else makes that differentiation, but I find it useful.
Note, however, that with "historic(al), the short form is the more specific; with grammatic(al), the long form is more specific. That is why I think we have something less than a rule here.
Student or Learner