I would imagine that it was politeness on his part, and who knows, maybe a bit of calculation.
There would be no way that the board wouldn't want him to retain that title. It's very likely that his role in the company will remain exactly as it is. It's the role best suited to him.
Of course, we expected Tim Cook to succeed him as he said the succession plan indicated. At least now we can stop seeing all the inane articles about how the board is looking outside the company for a successor.
Not to beat this one to death, but I don't get the politeness aspect of, "dear board members, I am resigning as CEO and now I would like to take Bill's position on the board." I don't imagine for a moment that this is actually how it went down, even if the resignation letter implies as much. Even making the letter public was kind of peculiar. More drama than necessary seems to be par for the course where Apple is concerned.
But yes, finally, all the succession talk will get behind us -- not that we won't have to endure endless speculation about "what this means for Apple." That talk will probably persist at least until mid-October (the next earnings announcement).