Yes, Volcarona evolves at level 59. The "evolution limit" imposed on Pokemon's levels is broken very frequently in the franchise, and that particular Volcarona is an example of this. There's nothing to understand other than the fact it exists and it's not a mistake. A particularly famous example of this is Lance, who uses under-levelled Dragonite in several different games, and also used illegitimate moves on several of his Pokemon in earlier games.
I guess what I'm saying is that the game will breaks its own "rules" on occasion, so you shouldn't be surprised when it happens. It might be easier to think of it this way: the rules the game has created only apply to the players. In exceptional cases, characters and wild Pokemon may breaks the barriers the games have established.
