If people don't pay attention to each other's actions and pick a number more or less at random, then yes, that is extremely unlikely. But if one person uses a formula that is based, for instance, on the average number from the last round, and sticks rigidly to this formula, and somebody else figures out what he does and takes advantage - then this is not unlikely at all. In fact, what we see in these two ponds is exactly what you would expect to see.