In XY, you can catch 454 different Pokemon, and (if I counted correctly) you can receive 133 other Pokemon through breeding and catching friend safari Pokemon.
Now pretend you and a friend live in some area in which your 3DS systems have no internet connection. For simplicity, let's say you can still somehow get every 20th anniversary Pokemon giveaway. That's 11 Pokemon. If you both have a different version, you can both naturally obtain 600 different Pokemon. That's 600/721, which is about 83% of the Pokedex.
Now imagine this same situation a few years back, in which you and your friend have a pair of HGSS games. Assume you can find your way to a Gamestop that can give you Celebi.
With the Safari Zone, Pokewalker, pokegear, and swarms combined, there is one Kanto Pokemon you can't obtain (Mew), 15 Hoenn Pokemon you can't obtain (Wurmple, Silcoon, Cascoon, Beautifly, Dustox, Taillow, Swellow, Nincada, Ninjask, Shedinja, Regice, Regirock, Registeel, Jirachi, Deoxys), and 46 Sinnoh Pokemon you can't obtain (Turtwig, Grotle, Torterra, Chimchar, Monferno, Infernape, Piplup, Prinplup, Empoleon, Starly, Staravia, Staraptor, Cranidos, Ramparados, Shieldon, Bastiodon, Burmy, Wormadam, Mothim, Combee, Vespiquen, Cherubi, Cherrim, Drifloon, Drifblim, Glameow, Purugly, Stunky, Skuntank, Riolu, Lucario, Rotom, Uxie, Mesprit, Azelf, Dialga, Palkia, Heatran, Regigigas, Giratina, Cresselia, Phione, Manaphy, Darkrai, Shaymin, Arceus).
that's 431/493 obtainable Pokemon in HGSS, which is about 87% of the Pokedex.
This means that even though XY has the largest amount of obtainable Pokemon, HGSS was still a more accessable Pokemon game for the time.
ORAS have 85 unique catchable Pokemon after soaring, 211 other Pokemon in the Hoenn dex, and 25 other legendary Pokemon to catch. Add the 11 20th anniversary Pokemon and the three announced XY mythical Pokemon distributions, you get 335/721. Roughly 46.5% of the national dex.
