Hmmm... maybe the same reason Groudon and Kyogre are not considered One-off Pokemon? (And if you don't believe that, you can either trade one or both of them to FR or LG and search in "ocean" Pokemon or "Plains" Pokemon, or just look at Serebii's pokedex.)
My guess is that you're the only person in the games that can see the Unowns. If you'll notice, none of the NPC's have them... They're special because they come from another dimension or whatnot. Forgive me if that's inaccurate. n_n; It's been years since I've seen the 3rd movie.