[HDThoreaun]

This isn’t true because of the weighting. If apple doubles in value SPY will be up 8% from that alone. It’s easy to see a scenario where one massive company has an incredible year and every other company returns below the average.

[vkou]

Or, alternatively, one massive company has a bad year, and everyone else comes out smelling like roses.

On average, 50/50 is a pretty decent benchmark here.