 An example from Edgington (2020) can help to figure out the difference. Assume a stack of 100 straws, out of which 90 are 10 cm long, 1 is 11 cm long, and 9 are 20 cm long. Consider the conditional “if the straw is longer than 10 cm, then it is shorter than 15 cm”. Assume similarity between worlds is set by the closeness in lengths between straws. The probability of the set of worlds in which Stalnaker’s conditional is true is \(\nicefrac{91}{100}\), since the 10-cm-straw worlds and the 11-cm-straw world are such that the closest world in which the straw is longer than 10 cm is indeed a world where it is less than 15 cm. But the conditional probability for the conditional is \(\nicefrac{1}{10}\), since only one of the straws that are 11 cm or 20 cm long is less than 15 cm.