Yeah, it's pretty obvious what they mean by Mythical. It's just saying that you can't find these Pokémon normally through the game, and you need events/giveaways for them. The reason Arceus, Victini, and Celebi are all referred as Mythical is just that. All other Pokémon that are legendary, like the mentioned Zekrom and Reshiram are availible through regular play, which is why they aren't Mythical. The List Goes:
Mythical:
Mew, Celebi, Jirachi, Deoxys, Shaymin, Darkrai, Arceus, Victini, Keldeo, Melotta, Genosect
Legendary:
Articuno, Zapdos, Moltres, Mewtwo, Raikou, Entei, Suicune, Ho-oh, Lugia, Latias, Latios, Regirock, Regice, Registeel, Groudon, Kyogre, Rayquaza, Uxie, Mesprit, Azelf, Dialga, Palkia, Heatran, Regigigas, Giratina, Cresselia, Phione, Cobalion, Virizion, Terrakion, Tornadus, Thundurus, Reshiram, Zekrom, Landorus, Kyurem.
Manaphy is tough to decide whether it is Mythical or not, i'll leave that up to you guys to decide. Please consiter this post, I think it is very relevant.