This is pretty common, actually. I wouldn't say it's the norm but yes, common. Many will take reservations as much as 2 or even 3 months in advance. Many hostels still take/keep records on paper/pencil in a reservation book. There are a few out there taking reservations many months in advance but I don't recall which they are.

Also, if some hostels say they're full when asking about dates far in the future, there's a good chance that they simply don't take reservations that far in advance. You could try calling those back about a month beforehand and ask again and a room might "magically" appear.

Good luck! Saludos, MadridMan