There is also a significant time factor involved. The longer a startup has been alive, the more likely it is to have a bigger exit. Startups that go to zero tend to do so fairly quickly. And startups that end up 10x take quite a while to get there (YouTube would be a major exception to this, reaching 1.6 billion in just 1.5 years).
The harder to predict ones are the ones in the middle. The less than 1x returns could happen quickly or take a decade.