To briefly try to explain why 3 and 4 are important, one way to put it is this. The cards in the hands of those who see the flop are NOT random. Their identity is influenced by the fact that they stayed around to see the flop. It's kind of hard to see why this matters at first, because it looks like there's no influence. But just because you can't see the influence, doesn't mean it's not there.


Let me see if I can give another example to show why 3 and 4 are important. Suppose you're sitting at a table, call it Table 1, where, for whatever bizarre reason, if one of the players holds suited cards, there is a 99% chance they stay to see the flop, while if they have unsuited cards, only a 1% chance.


Now, consider another table, Table 2. At Table 2, another bizarre phenomenon happens. Here, it's just reversed. If someone holds UNsuited cards, there's a 99% chance they stay to see the flop. If they hold suited cards, for some reason, there's only a 1% chance they stay to see the flop.


Now, imagine that both tables are dealt a hand simultaneously, and 5 players stay to see the flop. A 3-flush flops. At which table do you think it's more likely that someone has flopped a flush? It's no contest.


(Mathematically, what's going on here is just Bayes' Theorem. Sklansky mentions this result in some of his books, I think. It's a theorem that relates conditional probabilities.)