Quote from the book by Charles Poynton (2012, page 272):
“Tristimulus values are computed from a continuous spectral power distribution (SPD) by integrating the SPD under the colour matching functions.”
This is also detailed on Bruce Lindbloom’s math-page “Computing XYZ From Spectral Data”.
One SPD is D50, another is D65, so both will give different XYZ values according to this.
Hermann-Josef