I've pondered that question K99 and I reckon that the answer is that the calorie calculation is per weight. A large part of the weight of the grape is water (which has zero calories) while the raisin contains less water. Consequently, one grape might weigh the same as two or three raisins but the raisins would have more calorific content as they would two or three times the level of fructose.
sorry gigi but that's tosh. maxi29 has the answer, there are more raisins/100g than grapes/100g (as there will be less fluid) so inevitably more calories