ETFs are tracking the index, but the “amount change” is not identical between the index and an ETF, or even between two ETFs tracking the same index - as neither of them are the exact same things.
As a starting point, to demonstrate that all these things are not the same, ETFs often don’t have each and every stock of the index.
In an ideal world, all ETFs would invest in all the constituents of the underlying index.
VUSA is actually trying to do this, it’s called physical replication.
However, especially with particularly large indexes, think S&P 500 or FTSE All-Share, it’s not practical to buy all the stocks.
In the case of the FTSE All-Share, which has 650 constituents, investing in each and every one of the stocks is not practical.
The bottom 50 stocks in the FTSE All-Share make up less than 0.5% of the index, meaning the transaction costs involved in buying these stocks far outweigh the benefits of owning them.
So the ETF firm tries to build a portfolio with the same characteristics as the index with less securities to reduce transaction cost and/or the exposure to illiquid assets.
Another aspect is that often the ETFs tracking the same index are not even in the same time zone, e.g. VOO is traded on NYSE Arca, whereas VUSA is on the LSE (and a couple of other European exchanges). So they even have a different definition of trading day, different time windows to react to daily news.
ETFs tracking the same index also have different costs, which has an impact on how investors see them and invest in them, resulting in somewhat different supply and demand (and hence somewhat different growth).
So, again, the index-tracker ETFs are tracking, they are not identical to the index they’re tracking our even to each other.
This is an interesting article that details the replication issue, the cost issue, the issue of the cash drag that uninvested money coming from the daily operation of an ETF has, and other details that hopefully even help you pick the right ETF for your goals: