Maybe someone else will chime in, but those curves are generated with a sweep generator with a constant power output. Thus, the driver effeciency would be shown in the curve. If I am mistaken, then the charts are somewhat bogus.
While other speakers may be more "flat," those are a pretty darn good set of curves.
The Rat.
M80s, VP-150, QS8s,
SVS PC 20-39+, OPPO,
Onkyo 703s, Harmony 880
Sony 60" SXRD HDTV