I finally find out where the problem is from for one smart group I am using which does not show the correct number of watched listings.
n° of watcher = > 1
Status = active without offers or sales
When you move manually one (or more) watched listings from 1 folder to another one, the number smart groups shows decreases of the same number of watched listings you moved. A concrete example: I move 10 listings from folder 1 to folder 2. Before doing this the smart group shows 100 watched listings. Between those 10 listings I move there are 5 watched by someone. After moving them into folder 2, the smart groups shows 100 - 5 = 95. If you click on the smart group and then press OK the number returns 100, but if you don’t do this the number remains 95. The same problem exists also for sold listing.