Eanon, are you using the factory speedo????
I been trying to think of things that might change the RPM to MPH numbers. Many simple ones came to mind, like tire air pressure, but that gets ruled out because the factory speedo takes the reading at the tranny and not the tire. Air temp that might affect tire air pressure is then also ruled out. BUT.... if you are not using the factory gauge, tire press could explain it.
Now, if you are using the factory speedo, there is still a explaination.
Our bike has to use an electronic tach. The Analog (dial) tachs that tie into the ign coil are sensitive to resistance. That means a bad connection can change the reading. Corrosion or a loose wire can raise the reading. Bumping it to make a better connection could also lower it. On the small digital tachs, many use inductive pick-up (wrap wire around sparkplug wire). If that wire was bumped and the coils were tighter or looser, that can affect the reading , but only a tiny bit... Replacing the filter, did you remove the seat and tank? Or have you done that lately??? If a wire got bumped........
Just a thought.....