Why is it “geometric” but “theoretical”?
They're all adjectives, so how do you know when to use one over the other? Does it matter? I think it does...
I don't know that there's any general rule about how you would interpret adjectives that sound similar or have common roots.
In the examples you give, "historic" means that an event is important; "historical" means something related to history. So an "historical novel" would be a story about history, that is, a story set in the past. An "historic novel" would be a novel that is great and important. One might very well write a boring historical novel, i.e. a historical novel that is not historic; or a historic novel set in the present, i.e. a historic novel that is not historical.
"Instructive" refers to something that imparts insight or knowledge. "Instructional" describes something intended to be used for teaching. If you say, "Sally's over-reaction to the news was very instructive", you mean that you gained an insight into Sally's psychology from her behavior. If you say, "This book is instructional", you mean that it is intended to teach something, as opposed to being for entertainment or some other purpose.
In general, if you see such similar words, the best thing to do is look them up in a dictionary or at least study the context in which you found them to understand the difference in their meanings.
To summarize, historic has a connotation that carries importance; historical does not have that connotation, and more to do with age.
Sometimes it is easier to comprehend the difference between similar adjectives when they have nouns with them.
Further, according to the above website, OED: