Read an article saying modern milling strips too much from the wheat. They bleach it and remove the germ. That messes with the protein content. Your great-aunt's flour was probably weaker, which is actually better for a tender crust.
Have you ever tried baking with that old-school flour from a mill? I mean, what nathan said totally checks out. I used some unbleached stuff from a local farm last year and my pie crusts came out so much flakier. Idk, maybe it's just me but the store-bought kind always makes things tough. That weaker protein really does make a difference for tender pastries. It's like they've over-processed all the good out of it.