I asked myself why the differential (or offset) between Mash pH and Wort pH across the 12 Weyermann data points on the two charts posted above starts out narrower and then ends up wider, only stabilizing at an average of ~0.2217 pH points over the span of the final 6 data points. Then it finally hit me. This is a step mash. And for the first 6 sample/data points for which the differentials are noticeably lower than the average of 0.2217 pH for the final 6 data points, this is due to the mash we are observing being a stepping and rising tier of lower temperatures to higher across the first 6 sample/data points. And then finally for a stable mash temperature across the final 6 data points.
Then I asked myself again, why is Wort pH greater than Mash pH for the Weyermann data, and the only logical conclusion which I can fathom in answer to this question is that Weyermann measures mash pH exclusively at mash temperature.
And for this I asked why would Wyermann confound and confuse us by measuring mash pH at mash temperature, when all amateur home brewers (and the brewing book writers and brewing magazines which cater to them) conclusions point to this act of high temperature mash pH measurement being totally ludicrous and ill advised, and I realized that Weyermann are not amateurs, and they are not pandering to amateur misconceptions. And therefore they don't make the presumptive and multiple decades long collective delusional mistake of a collective group of amateurs and those purported experts who cater to them in the churning out of "worshiped" amateur level books and magazines, which all collectively lead amateurs to conclude that Mash pH and Wort pH must be identical, and in fact be one and the same, and that mash pH must be measured at room temperature.
Then I looked at what Weyermann concludes for their acid malt, with this being that on average every added 1% by grist weight of acid malt drops pH by nominally 0.1 points, and I asked, for which column between Mash pH and Wort pH does this data hold more consistently true on average, and that is clearly for the Mash pH column. And this lends weight to the conclusion that Weyermann measures Mash pH at mash temperature, and not at room temperature. And when they do measure at room temperature they are very careful to differentiate for this via calling it at that juncture Wort pH, and that is why they require 2 columns of pH results as opposed to the amateur home brewer expectation of only 1 column.
And lastly, Weyermann's 0.1 pH point drop per 1% of grist weight rule of thumb is only valid for pH's measured at mash temperature. It is still usable, though less precise and valid for room temperature pH measurement.
In all of this I'm not advising that you measure mash pH at mash temperature unless your equipment is certified to be up to the task. What I do advise is that when following peer reviewed expert from yore advice to best target mash pH at 5.4, you should seriously give consideration to adding 0.22 pH points to this pro brewer from yore target when measuring a room temperature Wort pH and merely presuming it to be the reading of a Mash pH.