These are misguided statistics. I can envision a scenario where using average numbers give you completely the wrong picture.

For example, the majority of the total value comes from the few outliers (AirBnb, Heroku, DropBox) and the other 80/90/95% of companies contribute relatively little to the total valuation.

So let's say they still own 6% of these outliers. Great! That's still worth a lot. But how much do they still own? Have their shares been diluted? Did they have to issue new shares due to further financing? Has that 6% turned into 3% or 1.5%?

I'm not an expert but it's quite possible that their initial 6% has been diluted.

I think the picture looks good, overall, for YC from an investment standpoint. I think avg. value is not a great metric by which to value the "average" YC company since my guess is that the majority of the value is on the top end.