Overview
Both expressions soften a statement, but they soften in opposite directions. とは限らない takes a generalization and denies that it always holds — “it’s not necessarily so,” poking a hole in a rule people assume is universal. ないことはない takes a denial and walks it back through a double negative — “it’s not that I can’t,” granting a grudging, reluctant yes. One weakens a “yes, always” down to “not always”; the other lifts a “no, can’t” up to “well, I could, I suppose.” The shorthand for students: とは限らない = “not always true,” ないことはない = “not impossible / I could, reluctantly.” The two are easy to lump together as “softeners” but do genuinely different jobs, so the boundaries below matter. The humor runs through varied mechanisms.
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