Can someone explain the logic of the grammar "とは限らない"?

Question

I found this example sentence in "A Dictionary of Intermediate Japanese Grammar" under a grammar point unrelated to "とは限らない": "運動をよくする人が必ずしも長生きするとは限らない。" I understand the meaning of the sentence ("People who exercise a lot don't necessarily live a long time.") but I'm struggling to understand the logic behind this usage of "とは限らない". If this dictionary did not have an English translation I would think it meant "Living a long time is not just limited to people who exercise a lot" since "限る" means "to limit" in other contacts. Without the "とは限らない", the sentence means "People who exercise definitely live a long time." From my understanding of "必ずしも", you can say "長生きしない" instead of "長生きする" and it should also have the meaning "People who exercise a lot don't necessarily live a long time". So why is "とは限らない" used this way? And bonus question, what is the semantic difference between "運動をよくする人が必ずしも長生きするとは限らない" and "運動をよくする人が必ずしも長生きしない"?

Answer 1

The expression of 「必ずしも + negative word 」means, not always(necessarily) so.

1.運動をよくする人が必ずしも長生きするとは限らない。translated into English,

People who exercise well do not always(necessarily) live long..

You can also say, or write

2.運動をよくする人が必ずしも長生きしない。It is the same meaning as above.

People who exercise well do not always(necessarily) live long..

The difference between 1 and 2,

1 uses 「限る」verb, which is [be limited ], it emphasizes the case of the longer life without exercise well more than 2.

We have more similar expressions with 必ずしも, 「必ずしも～わけではない」when you describe the reason and feelings. It implies it can't be helped, its something like that.

Answer 2

This is such a great question. To just address the last bit, とは限らない is used to imply that while sometimes possible, "x" is not always the case. So while it is possible that someone who often exercises may live a long life, it is not always the case (however often it may occur). As for the last query, I don't believe there is a difference in meaning, but the first is more common in writing as it has a softer tone. Additionally, negating the verb makes the meaning of the second sentence more confusing to the listener, I think.