This is where you'd have to get into the "books" to know him.
Unlike the show, the books are absolutely littered with clues... subtle clues, obvious clues... but GRRM has a list and he'll get to them ALL.
The books have been praised not only for their story but for their completion.
Martin makes almost no mistakes.
He's set up so many mysteries... half have already been played into (Red Wedding, for example)... but some have not.
For example, "show" watchers don't know about the "Three Dragon Riders" or Dany's visions in the House of the Undying (HotU)... but Martin sets these kind of things up all over the books and then DOES eventually play them out.
He CAN'T change tact at this point because he's set up the clues so thoroughly that he would be easily called out for making his first mis-steps if he does change the endings.
Simply put - it's something that the show-watchers don't know... that GRRM has this story so thoroughly set-up and established.
It ain't LOST... there aren't different endings that can be chosen.
There's one ending that he's planned... and when it's revealed, every subtle hint in the book... people will say "Ahhhh, yes... makes sense... I didn't see it... but yes, makes sense."
The story is going to go down as one of the best in literary history EVER... if he stays with his intended ending.
He knows this.
The ending won't be changed.
(He's deceptive in his answers, it's true... but that's more for the sake of throwing somebody OFF his intended finish. ex. fAegon.)