First of all, it probably depends on the type of screen. Secondly, it might be because 9f the brightness settings. Thirdly, the most used apps by the person. Some apps are better in light mode and vice versa. Fourthly, it depends on the way the person’s eyes are sensitive. For some, like me, white text bleeds into dark backgrounds and that makes it harder to read, but the dark mode feel and comfort make it much less straining in the end. Another reason could be the perceived sense of elegance which is just subjective with no explanation.