Real time systems including games have hard limits on how long something can take in the worst case, not the average. In games this manifests as: All the stuff has to be done in 16ms to render a frame on time, if we're late the user will perceive a glitch. The consequences can be worse in control systems in any number of real-world systems where a glitch can lead to stability problems.

So getting back to TFA, this is part of knowing what the "right" answer is for the context.