it is similar for me, penthasilia, you put it into words beautifully..... the space where i can "see" and "hear" what is being told to me. "readable" is a difficult concept for me to grasp. i would rather say: which deck gets the message through most directly/effectively/completely. and that does depend on whether i am able to establish a connection with the cards. and that, in its turn, greatly depends on my own state of mind in the moment i draw the cards.
the decks i connect most to at this time are the sakki-sakki, the fountain and the margarete petersen. but there are also decks that give me exactly what i need, even if i am unable or unwilling to listen. i tend to experience those decks as "unreadable", or rather: "unfathomable" at first, but if i spend some more time with the cards, or return to them in a later stage, the message sometimes suddenly comes through.
this happened to me with the silver era tarot, where i couldn't make any sense of its wimpy looking king of wands, until i realised that he might be telling me that looks can deceive and that fiery energy and visionary leadership might be more successful if built on a foundation of balanced and controlled emotion.
the mystic dreamer's seven of swords had me confused as well. couldn't connect to the idea of sneakiness and subterfuge attributed to that card at all. later on it suddenly made sense. that was me, there on the deserted tournament field, picking up the carelessly discarded swords, making sure they will not be used again by people wanting to hurt and kill other people, maybe taking them to a smith to turn them into something more useful.... it was a major breakthrough, enabling me to more clearly see and accept my role and function in the world at this stage of my life.
i only read for myself, so i don't know how reading for others changes a deck's "readability" but i would think it makes a lot of difference. as well as the way you are reading: do you meditate on the cards, tell stories, or do you consult the guidebook? do you use illustrated decks or pip decks? etc etc.