In much the same way that two 2.0L car engines can perform differently, two light sources claiming to be 840/4000k can present significant deviations in performance.
Like cars, the quality control of two 840 light sources can also differ. One lamp can be made to tight tolerances whereas the other may be an approximation of the advertised lamp properties.
The same applies to D65 light sources.
If you have multiple light boxes, this difference between quality and appearance of light sources can cause inaccurate results.
April 8th 2021