ugrant:
When you presume more than one condition and apply the presumptions to simple random odds, the combined conditions mean that the (separate) odds multiply together.
Example: the question, "what are the odds of flipping heads?" is one condition, and the answer is 0.5; but the question, "given that I flip a coin exactly two times, what are the odds of flipping heads both times?" the answer is 0.25 (which is 0.5 times 0.5), even though the odds for each flip is 0.5.
Why? Because the question PRESUMES exactly two flips (a condition), and asks the odds of getting heads on both (another condition); the conditions you presume mean that the odds (0.5) are multiplied together (0.5 times 0.5 equals 0.25).
In your question, the presumptions and conditions are:
IF
1) The second batter has a different best column from the first batter (because you've deliberately designed your line-up that way),
AND
2) the first batter gets a roll on his best column (because that's a condition of your question),
THEN
what are the odds that the second roll will be
BOTH (repeat: BOTH)
a) on a different column
AND
b) that column will be the second batter's best column?
The odds of "a" are 5/6ths (0.83); the odds of "b" are 1/5th (0.2) (because of the first "if" above). Combining BOTH these requirements creates 5/6 times 1/5, which equals 1/6.
You are arguing "But what if we presume the first roll lands on the first batter's best column, and the second lands on a different column? Isn't the only relevant probability that of 'b,' giving the answer of 0.2?"
No, because the only reason that the "0.2" appears in the equation is that you have ALREADY stipulated (in condition "a") that the second roll is on a different column. The "0.2" would not appear at all without condition "a".
It's like asking, "What are the odds of flipping a coin twice and having it come up heads twice in a row, IF THE FIRST TOSS IS HEADS?" The answer would be 0.5, but only because you have already stipulated the first toss. The answer without that stipulation is "0.25," which is 0.5 times 0.5.
The same goes for your question. The answer is 0.83 times 0.2, not simply 0.2. The number 0.2 is relevant only because you have already stipulated that the first and second rolls land on different columns, just as in my example it stipulates that the first toss is heads.
To sum up: You got the figure "0.2" because you started out by STIPULATING that figure. It's a prior condition, not a result. You started with a presumption, and concluded with your own presumption. You set the terms of the argument, and then answered the argument with the terms you set. That's tautology, not statistics.