The headphone.com measurement shows as an average, a substantially lowered treble as frequency rises, as I would expect in any diffuse field equalized headphone.
I don't know of any way to exactly measure a headphone so that you can directly correlate to a loudspeaker measurement, if that is the point here. Several factors are at issue when cross-referencing the two. However, if you want an accurate measurement of the transducer itself, you need to achieve a seal very similar to that on a real head to acquire the LF and mid response, and an acoustically damped baffle for HF response. Of course, this can not directly be used to correlate with what you hear, either, but does let one acquire data about the driver and rear enclosure system itself. The easiest method to get a measurement considering the ear structure and ear canal is to use an in-ear canal probe microphone, which will closely resemble the results gathered with the very expensive dummy head measurement systems such as used by headphone.com. Such an ear probe is outlined in the JAES paper, "On the Standardization of the Frequency Response of High-Quality Studio Headphones", Gunther Theile, JAES Vol. 34, No. 12, 1986, December, pages 956-969.
-Chris