"Experts" making such a statement usually assume the variable in question to be standard distributed – which is close enough to the truth for non-extreme outcomes. But a lot of variables do not behave like the perfect standard distribution at all: due to non-linear effects and self-enforcing feedback loops, their distribution deviates more and more from the standard distribution the more extreme their values gets (they are "fat-tailed"). Referring to the n-sigma of such extreme events is plain and simple stupid, because there's no such thing as a standard deviation (or variance) for a fat-tailed distribution.