5σ is not a rule. It's just a measurement of the probability that the result you observe is an accident. It means that it's very unlikely that it was just pure luck.

By the way, σ measurements are also used in many engineering jobs, and quality systems measurements. The kind of industry "goal" is to get 6σ (let's say when you manufacture consumer goods in large quantities, like millions every month). But for some other industries, 9σ is the benchmark - and from what I know, air companies have quality systems up to 9σ to ensure the lowest risk of something wrong happening.