How can Buddha take rebirth as Metteyya?

Gautama Buddha died and had attained final Nirvana. His cleaving to existing objects had ceased. His ignorance was destroyed. He had no upadana or karma left. Yet Buddha said that he will take rebirth as Metteyya . > And the Blessed One replied: "I am not the first Buddha who came upon > earth, nor shall I be the last. In due time another Buddha will arise > in the world, a Holy One, a supremely enlightened One, endowed with > wisdom in conduct, auspicious, knowing the universe, an incomparable > leader of men, a master of angels and mortals. He will reveal to you > the same eternal truths which I have taught you. He will preach his > religion, glorious in its origin, glorious at the climax, and glorious > at the goal, in the spirit and in the letter. He will proclaim a > religious life, wholly perfect and pure; such as I now proclaim." > > Ananda said: "How shall we know him?" The Blessed One said: "He will > be known as Metteyya, which means 'he whose name is kindness.'" In above quote it is clear that Buddha says another Buddha will rise. Is it not the rebirth of Buddha ? (I am not saying rebirth of Gautama but Buddha) My question is if Nirvana means cessation of birth then how is it possible for Buddha to take rebirth?