That number seems to be based on total valuations based on money raised. For example, if a YC company raises $1 million in exchange for 10% of the company then the company's total valuation is $10 million. However, unless there's a profitable exit strategy or YC sells its stake, YC gains nothing.
Too hard to make it a meaningful calculation. The average age of a YC startup is... what, two and a bit years? The average age of a startup from any other incubator is less, so the average value will be less.
You could break it down by year, but then you're into such small numbers of actual companies, with such a wide range of current valuations from zero to many millions, that it's an even less useful metric.