same output for your files
~/bin/get_colours.sh MadVRGrey.png JRVRGrey.png
patch | madvr | avg | step | jrvr | avg | step | delta
1 | 6,5,3 | 4 | 4 | 1,2,0 | 1 | 1 | 3
2 | 11,10,9 | 10 | 6 | 20,20,17 | 19 | 18 | -9
3 | 18,18,17 | 17 | 7 | 34,35,34 | 34 | 15 | -17
4 | 29,28,28 | 28 | 11 | 56,54,53 | 54 | 20 | -26
5 | 44,44,43 | 43 | 15 | 80,78,78 | 78 | 24 | -35
6 | 68,65,65 | 66 | 23 | 110,113,112 | 111 | 33 | -45
7 | 97,97,97 | 97 | 31 | 150,150,148 | 149 | 38 | -52
8 | 141,140,141 | 140 | 43 | 186,182,182 | 183 | 34 | -43
9 | 196,195,196 | 195 | 55 | 215,210,210 | 211 | 28 | -16
10 | 235,235,235 | 235 | 40 | 236,231,232 | 233 | 22 | 2
11 | 255,255,255 | 255 | 20 | 248,250,250 | 249 | 16 | 6
yours shows a much bigger difference between the two