And your GPS is probably only accurate to around 10m when stationary!
I wanted to check the top speed of my Fireblade. The digital clock went as high as 299km/h (max display on some digitals) and I was convinced that the bike couldn't do 299.
So I fired up a GPS unit, stashed it securely under the seat and headed out to an "undisclosed location".
After a few runs at 299, stopped and reviewed the GPS....max speed achieved = 288km/h which is about a 4% error. Your figures are around 6% so we are probably very close to the truth.
Haven't checked the NC yet, but I doubt it will hit 288!