I'll just go out on a limb and say that upgrades aren't normally-distributed random events. They're driven by algorithm.

So I may be 0 for 100 and you may be 10 for 10, and luck (or distribution of random events) has absolutely nothing to do with it.

In a similar vein, it's comparable to how data about IDB or lost baggage is lumped together to make those events appear incredibly rare and randomly distributed across all events, when in fact they aren't.