CRT image is an analog signal so it always has those little imperfections, especially as the screen ages and phosphor matrix gets burned, and there's dust settling so all dissipation curves change slightly, and each analog electronic component on the board had it's own tolerances that changed over time, and then EM noise from environment and power grid was also leaking - all of that slightly affected timing and precision of the beam. It's not just that it's super hard to digitally emulate that, but even back then no 2 CRTs were giving 100% the same image, especially after a few years of being in use.