Like you I have now carried out the test multiple times (4 if I'm honest). My experience is similar to yours with slightly different readings at different points of the picture. I have four different sets of results but in none of the tests has the "correct" exposure produced a reading of 128 (plus or minus 15). I have found through changing the setting on the camera in 1/3rd of a stop increments the nearest I can get to 128 is at an exposure between 1/3rd and 2/3rd over the metered reading.

Am I being completely clueless or does this ring true for some others?

Cheers