According to the Car Talk guys, engine break in isn’t really a well understood phenomena, manufacturers tend to guess (educated guesses, mind you, but still guesses) as to what the correct period of time. That, of course, doesn’t answer your question as to why the mileage has increased.
I can hazard a few guesses, however. One is that aerodynamics have a greater effect on fuel economy the faster you go. So, assuming you’re crouching down over the fuel tank when you take the bike over 55 MPH, and you’re now driving over 55 MPH more often (on many automobiles the break in period requires you to drive the vehicle below 55 MPH for a number of miles I assume there’s similar advice for cycles), and you don’t lean over quite so much at lower speeds, you’re effectively lowering your wind resistance. Another possibility is that because the rings have finally seated themselves properly, you’re getting more efficient combustion of the gas. Finally, it could be the deposition of carbon in the cylinders is increasing the compression ratio slightly, and thus enabling you to get better mileage.