Retro Video Game Graphics: Redrawn vs Converted

[ZetTheLegendaryHero]

I  would like to know why some retro video game graphics were redrawn, while others were converted. I mean not only, in the case of from platform to platform, but in the case of the same platform. For example, Super Street Fighter II on the Commodore Amiga  redrew the graphics to be smaller, and Super Street Fight II Turbo on the Amiga used arcade perfect graphics.