There is definitely something to be said about the quality of pictures taken in photo booths, but the study specifically used pictures that were all of sufficiently high quality:

> Photos of politicians were collected from parliaments around the world. The gender and skin tone were recorded for each photo using the Fitzpatrick skin-tone scale. Each image met the Home Office standards for a passport photo.

That definitely suggests the software used for automatic image quality check simply isn't up to par.