posted: Feb. 1, 2008 @ 7:27p
Also, index funds generally do not track the market capitalizations of the funds "exactly". To do so would incurr enormous trading costs. So they allow some deviation from the "true" index
Finally, especially in the case of indexes covering large numbers of companies, the index fund may not contain every single company.
These two cases may be accomplished with some sampling methodology to match the "model" of the index. If you notice two index funds "tracking" the same index may have some variation on returns that is not purely explained with differences in expense ratios.