why is this called a DIGITAL DIFFERENTIAL ANALYZER?

(defun lineDDA (xa ya xb yb)
(let ((dx (- xb xa)) (dy (- yb ya))
steps k xIncrement yIncrement
(x xa) (y ya))
(if (> (abs dx) (abs dy))
(setq steps (abs dx))
(setq steps (abs dy)))
(setq xIncrement (/ dx steps)
yIncrement (/ dy steps))
(setPixel (round x) (round y))
(setq k 0)
(while (< k steps)
(setq k (+ k 1))
(setq x (+ x xIncrement)
y (+ y yIncrement))
(setPixel (round x) (round y)))))

difference is any product of subtraction.

analyzing is applying the rules of logic to some process

your code obviously does subtraction and applies some logic to its result.