I think that is a combination of the light background reducing the contrast (as you are concentrating on the text not the background it appears that the text is closer to white rather than the background being further from it), and a fair number of the pixels not actually being true black anyway due to font-smoothing trickery (more so if you are using a non-high-DPI display and an OS that is using a font renderer which prioritises strict shape accuracy over snapping to the pixel grid to reduce blur).