An index fund promises to match an index rather than beat it. Delivering that promise is an operational problem, and the residual difference between fund and index has identifiable sources.
Full replication holds the whole index
The most direct method buys every constituent in the same proportion the index assigns, so the portfolio mirrors the benchmark exactly at any moment.
This works well for indices of large, liquid companies where every holding can be bought and sold without moving its price.
It becomes impractical for broad indices containing thousands of names, some of which trade rarely and would be expensive to hold at accurate weights.
Sampling reproduces the characteristics instead
Optimised sampling holds a subset chosen so the portfolio's exposure to sector, size, region and other factors matches the index closely.
The fund accepts small deviations on individual holdings in exchange for far lower trading costs, which usually improves the net result.
The trade-off is that sampling introduces tracking error whenever the excluded holdings behave differently from the ones standing in for them.
Rebalancing is a scheduled, public event
Index providers add and remove constituents on announced dates, and every tracking fund must adjust its holdings around the same time.
Because the demand is known in advance, other market participants can position ahead of it, which can worsen the prices the funds receive.
Fund managers mitigate this by trading around the event rather than exactly at it, accepting slight deviation to avoid the worst of the price impact.
Cash and dividends create timing gaps
An index is a calculation and holds no cash. A fund receives subscriptions, pays redemptions and collects dividends that sit uninvested briefly.
Any cash balance means the fund is slightly under-exposed, which drags in a rising market and helps in a falling one.
Some funds hold index futures against pending cash to stay fully exposed, which reduces the drag at the cost of a small additional expense.
Securities lending offsets part of the cost
Funds commonly lend holdings to short sellers in return for a fee, and that income can offset a portion of the expense ratio.
The practice introduces counterparty risk, managed through collateral requirements that typically exceed the value of the securities lent.
How much of the lending revenue is returned to shareholders rather than retained by the manager differs between providers, and it is disclosed in fund documents.