I actually always see around 70-75% attenuation, regardless of the mashing process just recently I made a Belgian Blonde with an intense step mash of 30 minutes at 64°, 20 minutes at 67° and another 20 minutes at 72°. Finished at 74% attenuation, in spite of a small dose of sugar. The temperature is correct (or identically incorrect across several thermometers, which I find unlikely).
Quite possibly, my hydrometer is off, but again: I've had a couple of them (I'm clumsy, they break easily, what can you do) and it has been the same throughout. I've had it when I was mashing in a direct-fired pot, and I still have it on my grainfather.
And yes, I do treat my water. I've never actually measured my mash pH, though, relying on estimated values and municipal water reports.
Nonetheless, I call it a "phenomenon", not a problem, because the low attenuation is not reflected in the flavour. And it means less alcohol, which is a good thing in my book.