I've seen a very similar problem to this referred to as "black swan events"[1]. The whole point is you can't actually compute it. You see the period, it's there. What he's doing here isn't science, it's a guess.
As others have pointed it, it's rather rare that events happen with perfectly distributed probability (probably the opposite). For instance, the chance of being in an accident is much higher if an accident just occurred right next to you. It's almost way more likely to get sick, if others are already sick. In fact, although I don't have any statistics to back me up, I'd guess that most events happen in clusters (including spelling errors, or when you test for perfect pitch in children, when you go to the music class).
This is essentially a guess, and it's better to say you don't know than guess wildly.