Hanning window correction in Tonality analysis #255

I am confused about the Tonality class in iso_1996_2_2007.py.

On line 34 of iso_1996_2_2007.py a correction for Hanning windows is defined:
_WINDOW_CORRECTION = { 'hanning': -1.8, }

This correction is subtracted from the total level of the sum of all lines of the tones in the cricital band on line 505, so effectively 1.8 dB is added:
tone_level = tones_level(tone_lines) - window_correction(window)

I understand the reason for a 1.8 dB correction is explained in the note after equation C.1 in ISO 1996-2: 2007:

NOTE If a 'tone' is a narrow band of noise, or if the frequency of a tone varies, the tone appears as several lines in the averaged spectrum. In such cases, the tone level, L_pt, is the energy sum of all lines, with levels within 6 dB of the local maximum level and corrected for the influence of the applied window function. (For Hanning weighting, this is the energy sum of the lines minus 1.8 dB.)

I believe the same correction is applied when calculating the total masking noise level in equation C.11, calculated with the function on line 78:
return dbsum(noise_lines) + 10.0 * np.log10(frequency_resolution / effective_analysis_bandwidth)
When using a Hanning window this is roughly equivalent to:
return dbsum(noise_lines) - 1.8

So to summarise I would expect on line 505 the -1.8 dB should be added to the total tone level. I hope somebody can shed some light on this. Any help would be greatly appreciated!