This is what I got from the website.
D'Addario Flat Tops are round wound, then precision ground, leaving the outer surface smooth and "semi-flat." This produces a smooth, flattened playing surface, but does not sacrifice the superb tone and sustain associated with phosphor bronze strings. Flat Tops greatly reduce finger noise and are excellent for recording and flatpicking.

All that I can say is that they really ring for me but that's on my guitar so it may well be different for someone else.