Update, brewed my first batch using Bru'n Water. 10 min into mash, pH was 5.6 so I decided not to screw around with it. Measured pH 20 min later, 5.3
Looks like Bru'n Water did a very good job for me
I don't know why this tendency occurs, but my observations show that mash pH tends to drift toward a pH of about 5.4 over time. If it starts high and you've implemented water adjustments that should have produced a lower pH, the pH does tend to move in that direction with additional time. A similar thing seems to occur when the initial pH is low and you've implemented water adjustments that should have produced a higher pH, it rises over time. I assuming there is some sort of delayed chemical reaction occurring.
The bottom line is that if you have implemented proper pH adjustments based on accurate water information, the predicted pH is fairly likely. If a measurement is off, wait another 10 or 20 minutes and recheck.
I do have one caution, if your water mineral and acid additions aren't uniformly mixed into the WATER before adding the grain, you can easily have variation in pH across your mash bed. Add everything to the water first and mix it well before adding the grain. That is one less thing to worry about.