A couple of possible reasons:
A disproportionate amount of stock-market risk (e.g., scheduled economic and earnings releases) happens outside market hours, with the goal of avoiding destabilization of the market. (Extended-hours and futures trading are typically occurring, and show the immediate impact, but the regular-hours SPY does not.) Thus, overnight ...
Per the request above:
The market reacts to overnight news which includes after hours earnings announcements, economic reports, overseas trading, etc. (very, very few companies announce earnings during regular hours trading). Because of these, the market tends to gap up or down in the morning. In a long term bull market, those gaps are net positive
The following items could play a factor:
Intraday margin is higher (e.g. 4x leverage for intraday but 2 x for overnight), causing people to buy at the open (push the price up) and sell at the close (push the price down). While people could also use intraday margin to create short positions, in general there are more long buyers than short sellers.
The S&P 500 is just an index - a list of specific stocks, it is not an investible instrument. What you are finding are some ETFs that track the S&P 500's returns, as well as ETFs that track other indices (like the Dow Jones US Index) and other variants or segments of the S&P 500 (e.g. XBI is specific to Biotech).
Of the ones you list, SPY, IVV, ...
S&P publishes at least three S&P 500 indices:
Price Return S&P 500 index — this is the one you see on TV and financial news. Common symbol: SPX.
Total Return S&P 500 index
Net Total Return S&P 500 index
Similarly for the Dow Jones Industrial Average (DJIA). The Price Return DJIA is the one you see on financial news.
According to S&P ...
You are assuming that a stock will jump after the inclusion into an index is announced. However there is some issues with your assumption:
Inclusion is not immediate. For many indexes there is a time between the announcment of the inclusion and the actual inclusion
People might expect inclusion of certain stocks and buy them to bet on an index inclusion ...
All ETFs tracking the same index will have roughly the same performance. There is always a small tracking difference but whether this tracking difference will turn out in your favor or not is unpredictable and over the long run it will even itself out. Therefore comparing ETFs based on past performance is pretty pointless.
Instead focus on the following ...
When a stock goes ex-dividend, its share price is reduced by the amount of the dividend, creating an equal and opposing capital loss. Ignoring taxation, if received in a non sheltered account, in order for that dividend to become a gain, the stock must rise by the amount of the dividend.
IOW, if you buy XYZ at $100 at the close and it goes ex-dividend ...
Only SPY, IVV, and VOO are "true and only".
If you are holding it for years, they are equally "stable, reliable, consistent".
If you sometimes buy/sell at extreme volatility, e.g. Market Order on Open, SPY is more consistent intra-day, at slightly higher expense.
The root cause of this is probably the "Rule against perpetuities" - essentially, in many jurisdictions, you can't write trusts that last indefinitely.
The rule is that
No interest is good unless it must vest, if at all, not later than twenty-one years after some life in being at the creation of the interest.
So by picking several people who are very ...
There was an article about this in Bloomberg not long ago. The short story is that pegging the trust termination date to the end of life of those eleven persons allowed for a more distant termination date (keeping in mind that unit investment trusts need a termination date).
The Fate of the World’s Largest ETF Is Tied to 11 Random Millennials
The goal of the ETF is to follow the index as closely as possible.
They would manage the rights in accordance with how the index handles the rights. If the index is adjusted to include the rights then the ETF should hold those rights as well.
The specifics should be in their prospectus, or if there are specific cases, the ETFs investor relations department ...
As you discovered, the S&P index returns, if calculated just by looking at the index year on year, do not account for dividends. It only goes back to 1988, but ^SP500TR can be useful for what you seek.
I'd add to your idea of treating the index as if it were a stock, an S&P unit or share. Each year, don't adjust the index, adjust the number of units. ...
The question asks about changes in components, i.e., additions or deletions. This is reconstitution, not rebalancing, which merely changes the number of shares of existing components in the index. The S&P 500 is derived from their Total Market Index, which is reconstructed once a year. Rebalancing does occur quarterly, as stated.