He's a performer who is good at creating catchy character concepts and developing them. It's really not too different from how a good writer is able to write for multiple characters, or how a good actor can play different and convincing characters in different films.
The only difference is that, unlike most, Myers plays multiple characters in the same film.
To be honest, although the characters indeed "feel" different, once you've seen the films quite a few times (and especially if you watch behind-the-scenes materials), you start to recognize Myers in each character. Not only physical characteristics (his face is chameleon-like in that it is a great canvas for makeup/prosthetics, but you can still see certain defining features like his eyes and smile subtly coming through), but in his personality and sense of humor.
It's part of why I was able to go along with the "Austin/Dr. Evil are brothers" twist easily in the third film.
And FURTHERMORE, this is my signature! SERIOUSLY! Did you think I was still talking about my point?