Over time, the membership of securities in the index will change, and frequently so due to an arbitrary decision from a management committee rather than as a result of an explicit formula. (alpholio.com)
Additionally, some enhanced index funds use complex mathematical models to identify the strongest securities in the index or look for price inconsistencies it can potentially capitalize on. (capitaloneinvesting.com)
The active / passive mix does have effect on the relative prices of securities in the indexes versus outside the indexes. (alephblog.com)