>>14306273This. There are 40 empty spots in that dex that could easily be hard to find pokemon. A lot of the leakers have been speed-running to try and get as much info as possible, but I can remember running around in one patch of grass for a long time trying to find that pokemon that only shows up 10% of the time. There could also be certain ones that require specific things to appear, like carrying a particular item, or having a particular ability. In a generation that has evolution methods like turning the DS upside-down or having a dark-type in your party when you level up, I can see there being a few gimmicky-to-find pokemon.
Also, when we just had Victini as #000 in the Unova dex, we can't rule out weird dex number things happening, like a few more pokemon after Zygarde that require event items but aren't necessarily legendary. Plus there may well be a trio/duo/sprite pokemon that require event items or must be obtained from an event. I may be wrong, but until I see an official game guide filling in those spots, I'm not gonna assume anything.