One thing I've noticed from this forum (and the Northern Brewer forum) is that targeting a lower mash pH seems to be the trend these days.
My guess would be this has something to do with the rising popularity of Bru'n Water, which states the recommended mash pH range is 5.3 to 5.5 (as measured at room temperature).
I know through experience of others , and then my own certain styles just taste better to me mashed at different PH. I like a wit or krystalweizen or fruit beer around 5.2. IPA or pale ale at 5.4, and 30+SRM dark beer at 5.5-5.6.
There's definitely several things going on here. The first is the pH of the mash itself and how it relates to conversion. From what I understand, pH in the higher end of that range (~5.6 measured at room temp) leads to the best conversion in the mash. Secondly, there is the consideration that must be taken to keep the pH of the sparge from rising too high in order to minimize tannin extraction.
Finally, there is the pH of the finished beer itself and how it relates to flavor. The numbers you're talking about really have more to do with this than the pH of the mash itself. Even though you're setting a specific mash pH for these beers, what you're really doing is targeting a
final pH to get the flavor profile you're shooting for. For example, you like your wit's to finish with a lower pH than your dark beers. You're choosing to adjust your mash pH, which will result in a lower kettle pH, and a lower pH after fermentation. You could just as easily take a measurement of your finished beer and adjust the pH at that point, even if your mash pH was actually 5.6 instead of the 5.2 you normally shoot for.