Why does Jim think Silver must be disappointed by the captain's map of the island?

Because of his knowledge, Captain Smollet asks Silver to look at a chart and identify the place the ship should anchor. Jim, astutely, recognizes that Silver merely wants to look at the chart in order to find out where the treasure is buried, but luckily, from Silver's disappointment, it is clear that the mark that Silver looks for was not on the map.