It’s worth remembering that the artists were authoring the sprites on workstations with CRT displays, too. Which is to say that the images on the left are probably closer to what the artists themselves were seeing as they worked than the ones on the right.

Not to try to minimize the skill of the artists, but I think what you’re seeing in these examples is as much due to display technology (both consumer and professional grade) being fundamentally different at the time than it is today, as it is due to anything else.