Jon:

Petrie had several addition mistakes in his work, but only fractional...nothing of great consequence. However, he does measure more than one set of corners.
He does not reference other works before him since he was employed (paid) to measure the site, not to simply compare notes.
It is the variations and lack of detail from previous works that provoked the man into designing an accurate means of measure…the graph you refer to is unique…a brain-storm of an idea that Petrie composed.
You have overlaid both sets of measure showing the slight discrepancies, but it is very deceiving.
I was impressed with Petrie's plotting of the layers years ago. After studying the course thickness and comparing to the "first" layer I decided to "expand" upon his “expanded” measures. There was something "odd" about the results, they are shown below.


Note the angle of 56.7 degrees...it’s a 3:2 tangent ratio (56.31 degrees is perfect). This line (red line) is derived from the alignment of the few “extraordinary” thickness of course measures.
Compare the linear height of the #35th course (56.4 Rc) with the numerical value of the angular measure…look familiar?

Best.
Clive