How could they be so far off?

We can, with all accuracy, suggest some speakers are bright while others are not. Or, some speakers have bass or they don't. Or a recessed or forward midrange. This leads one to conclude there's little to the notion of flat frequency response. For, if it were flat, there'd be no discussion needed. Take a pair of B&W 800 series—a speaker that is one of the most popular in the world within the medium to expensive range. For a brand of speaker to be so accepted it has to be a good loudspeaker. And it is. But they—like every brand of loudspeaker—have characteristics consistent with a deviation from our picture of flat. In the case of the 800s some might say: ultra revealing, overly accurate, a touch of too bright. Others say they are perfect. We might get just the opposite when looking at another brand: too dull, midrange heavy, bass shy. Or, perfect. What's important about this observation concerns flatness: there's no such thing. Not with published deviations of +/- 1.5dB for great designs and broader for the less accurate. And yet the flattest sounding speakers are the ones that do not draw attention: The few with sculpted response that raises no red flags and points to nothing but musical qualities we have come to accept as flat (despite their measured deviations). In fact, all speakers are far from flat. It's just how close designers get to what sounds flat that determines their performance. They are all off.