Well, I don't know but could only guess. The speed is supposedly set by a combination of the media and what your burner says to do with that media. It would therefore seem logical that the exact same media would get the same results.
Perhaps the media is being read differently each time. Do you have any other specifics about the media you might be able to compare each time? Are you opening the drawer prior to retesting the media? Are you restarting Nero before each test, or could some information from a previous test be stored and interferring with the next test? Nero may need to burn to discard previous information.
If you test 5 of the same media, are the results about the same as testing the same disc 5 times?
Have you checked your log to see if the media shows up the same each time you burn?
That might be a good question to ask Nero.