I use Bru'n Water to predict my mash pH, in conjunction with actually taking pH readings using ColorpHast strips. As long as I use +.3 pH correction with the strips, they support Bru'n Water's predicted mash pH 90+% of the time.
Today, I took a pH sample from my IPA mash and the strips indicated a pH of 5.6-5.7 (I almost always shoot for 5.3-5.4). I was sort of skeptical of this reading, so I tried a second strip. It ended up reading the same thing. Out of curiosity, I took a sample of the first runnings and it was dead on at 5.4. With my limited tech/science knowledge, I can't explain the discrepancy here. Can someone explain why the sample from the mash might have been higher than the that of the first runnings? Which one should I trust as an indicator of mash pH? At this point, I'm more inclined to trust the pH of the runnings sample since it supports Bru'n Water's mash pH prediction. But maybe the pH of the first runnings should be less than the mash pH?
edit: One more thing -- I brewed almost this same IPA recipe a few months ago with 1% acid malt. The mash pH was dead on at 5.4. The only things I changed for today's beer was using a slightly darker English cara malt (approximately 17L darker), and 4% aromatic. According to Bru'n Water, those changes resulted in a .1 pH drop, so I didn't use any acid malt this time (I use the acid malt ROT that each 1% drops pH by .1).
edit 2: Edited post to clarify that I'm referring to mash pH.