> why is the text color set slightly off black

Because pure black causes eye strain. Dark gray on white is superior for long reading sessions when your paper is white. The contrast really hurts after a while if you do pure black on pure white. This is a known phenomenon.

In fact, there's experimental evidence (https://www.nature.com/articles/s41598-018-28904-x) that this high contrast plays a hand in the onset of myopia, which in extreme forms is correlated with glaucoma and other vision disorders.