DW1000 maximum output power

According to the datasheet, DW1000 has a maximum power spectral density of -35 dBm/MHz. However, by setting the TX gain register for maximum gain, i measured a spectral density of -27dBm/MHz. I have followed the procedure in “DW1000 DEVICE DRIVER APPLICATION PROGRAMMING INTERFACE (API) GUIDE”, section 6.3.17 and used a second Spectrum Analyzer to make sure there is not a problem with the instrument.

Has anyone found something similar? Is this even possible?