Just a quick question.. Why does Dawud's hadiths skip here and there?

Like for example, there might be hadith number 101,102,103,104 but then it skips and goes to 107, leaving out 2 hadiths! Not only that, but on all sites, it shows "partial collection of Dawud."

Were some of them taken out because they were not authentic, or does anyone know why? It has always bugged me!