I am curious about this myself. I always thought it was a hysteresis thing. Hysteresis, is a tendency for magnetic media to retain a recording. If you write over it once, and you know the nature of the media, you can figure out if there was a 1 or a zero underneath it based on small deviations from the value it should have been if there was never any data there to begin with. This gets shrunk dramatically the second time, and now instead of two possible values around a known constant, you have two possible values around two possible values. It just gets fuzzier after that.
Those values by the way, are pretty darn small to begin with -- one requires extremely sensitive equipment to perform data forensics.
And yes, they don't write constant values on any drives any more, they write patterns when they're reformatting, because it makes for a better hard disk anyway. "Like new."