Pension funds are managed by a consortium of instructional portfolio managers- 401k assets are not! Instead participants are given a menu like assortment of mutual funds to chose from. The performance of a plan is directly related to how it's non-sophisticated participants allocate their money so it's no surprise the Pros beat Mr. Joe Schmo. Furthermore, pensions can invest in alternative strategies like PE that a 401k can't.
Furthermore, the only way to measure outperformance is on a risk-adjusted manner.