> The Room RT60 is very high. Not the best situation for this type of measurement.
> The distortion of the measurement is very high.
> The mic distance is too far unless the speaker is elevated very high in a very high ceiling room.
> The window is too long for the mic position assuming a typically room size and normal speaker height if you used the default Turkey 0.25 filter.
More to your question:
The minor sag above 2k may be due to the elevation of the mic vs the speaker. Some measure on the TW axis and some on the bisector of the TW and MW. You will find significant SPL response differences around the XO for different elevations. It is unusual to measure on the MW axis, but I suspect a manufacturer picks the most advantageous situation.
Assuming your TW is about 36" above floor, I would suggest you move the mic to about 40" from the TW and position it on axis. Window with the left window set to 0.4 ms Turkey 0.25 and the right window set to 10 ms blackman-Harris 4. With these setting the values above about 500 Hz should be reasonably accurate.
Better yet measure outside and try different mic elevations until you get the expected result. Note that from my memory Stereophile may have taken an average of 5 measurements (on axis and 15° up, down, right and left). You will have to investigate and do the same to expect to get the same result.
In the end, the minor anechoic SPL variation is insignificant compared to the room impact on the response.
Before you do any of the above first investigate and correct the reasons for the high distortion. I don't see a soundcard cal file loaded so you will want to start there to assure that the measurement system is working properly.