My Boss or the Man would can't identify me

Well I guess I sort of lied in my mission statement by saying that I would not post about my boring job and/or daily life, but I just feel that this story is too good to pass up. I hope to make this post short enough so that those among us who get ass cramps from sitting in one space too long will be able to muddle through without any emergency surgery being required (and you know who you are).
I work in a typical office environment, office gossip, office politics (my main boss is an elected official so it get worse at certain times), and all the other good things involved in living life in a cube. My boss is a politician now that is not a good or bad thing as far as I am concerned, but simply a fact. He has to know a lot of people and has a lot of people working for him. That is said in his defense. Now, I have worked in my office close to three and a half years. I have not had a lot of person to person contact with my boss, he works on a different floor and he is the boss he does not need to know what is going on in my world on a day-to-day basis. However, one day not so long ago as I was in a co-worker's office my boss came in and starting speaking to me about a work related issue that he thought I was "handling" sadly for the both of us, I realized quicker than he did that he has mistook me for another co-worker who was actually handling the issue he was speaking about. Now this is a tricky moment in a peon's life, how does one gently tell one's boss that he has mistaken you for someone else, someone who has been with the office a year less than you, and you really would like to bash his face in for being such an ill-formed soul. I merely said "no, boss you have me confused with someone's else, he is a little shorter and tubbier than I am and works in a different division". My boss looked flummoxed by this news muttered something, and walked away as laughter burst out behind him mostly at his expense, but some at mine as well. The fun twist on this cat's tale is about two weeks ago my boss and the powers that be offered me a "promotion". They were amazed when I turned it down. Wonder why?