This technique has been used for years to create an album that "pops" on the radio. Since most people have crappy stereos (like in their cars), by dynamically cranking up the volume in the mid-range, you can make the song sound fuller. The problem is that for anyone that cares about quality and/or has some decent equipment, it sounds like ass, since all the loud parts have been clipped and all the quite parts have been amped up, and all the highs and lows have been nerfed.
Originally it was a style done with music production methods, now it's just done by some yahoo with CoolEdit Pro in post-production. Look up "wall of sound":
http://en.wikipedia.org/wiki/Wall_of_Sound
But it wasn't a "mistake". No way a huge album goes out the door without a few audio engineers reviewing it. They made a conscious decision to do this to it so it plays better on the radio.