Martin having given them an end-point outline will undoubtedly have left miles of wiggle-room. All he has to have done is say "this person ends up on the throne" and they can take wildly different routes to get there.
Which is actually good for both sides- if there are differences between show and books, this means there will always be surprises for the audience who has experienced the series in the other medium.
Perhaps even greater differences than this. Now that Martin has the rumored 8th book planned, and several plot lines now diverge from the books, I can only see the differences growing over the next few years.