David, unfortunately the test image you've made available isn't correct.
Load it into PS and check the actual data and you'll see lots of inaccuracies; patch #2 = 1,1,1, Patch #13 = 15,15,15 patch#234 = 238,238,238 etc.
As the test is designed to pin down exact levels, to measure errors of 4 or greater make it not as useful as it should be.
Possibly this is a result of JPG compression, but a DIY uncompressed tiff isn't too difficult to make anyway.