this chart just shows that virtually all analysts come up with basically the same numbers with minimal variation, because analysts have learn that its ok to be wrong, as long as you are all wrong together. (just eliminate the highest and lowest call and see how much closer the mean and median get).
Furthermore, their similarity is probably because they all get expected earnings (aka 'consensus earnings') from the same source, and they are applying their preferred P/E to the expected earnings to get their target S&P level
Analyst take their numbers from the company CFO and processed it via their systems. Every house has a slightly different system or methodology but the outcomes should not be drastically different. My general rule is never to buy when the last analyst turns bullish and vice versa. That said, I do think that the best time to buy is when the last analyst turns bearish... it just shows that the share price has already reflected all the bad news and chances are the price may have bottomed.
Having worked in markets, there has been constant whispers of the house taking on a position prior to release of such reports - so essentially, they have bought and would wish for others to buy so as to push prices higher or for them to sell into strength.