Nirvana is the well known to be a certainty, which means that if you feel the need to verify that you have the fruits of it, you do not have it...
Any other result are got by induction, over at least space and time (since most people choose to think through these categories), and therefore cannot yield certainty. People feel the need to verify, more less individual by individual, these kind of results, precisely because they are not sure that they hold beforehand. Thus they verify that the result hold statistically, with personal margins of statistical significance, to better choose to claim, out of laziness (or pragmatism (to sound less adhering to subjectivity), for those who choose to see themselves as pragmatists), that if the result holds, according to a few individuals, a few times, it holds more than a few times (sometimes forever, without statistical margins as in mathematics, for a few philosophers of mathematics (well, until somebody with good reputation claims to have found a mistake in a accepted proof old of a few years !)) and for everybody, therefore fulfilling their dream of truth by universality and necessity. Of course, they choose not to question their clinging to the concept of universality and necessity.