The one in the middle has the robber turning up only 6 times in the entire game.
Throwing two six-sided dice produces one of 36 different outcomes, but because Catan uses the sum, the resulting totals from 2-12 have different probabilities of occurring, as shown above. There is only one way to roll a 2, but 5 different ways of rolling a 6. In the game, the numbers have one to five pips on them, and these represent the chance in 36 of that particular resource being produced in a given roll. However, a typical game involves a finite number of rolls, and the distribution is not going to match the above perfectly. Sometimes, there will be deviations from the averages that will outrage players. More 12s than 6s in a game? No 5s in an entire game? You'll see these and worse. The second picture shows 9 simulations of 72 rolls (a fairly average number for a game), with the red dots showing the predicted distribution and the blue bars the observed values (made using Excel using RANDBETWEEN and COUNTIF functions. This simple spreadsheet is appended to the end of this step if you want to play around with it; just hit "save" if you want a new distribution). All sorts of deviations are seen; as examples, the "game" at top left features 2 being rolled as often as or more than 3, 5, 9, 10, 11 and 12. The one at bottom right has 10 being rolled more than 6 and 8 put together. The one in the middle has the robber turning up only 6 times in the entire game.