I don't think any rhyme or reason will ever be found in those changes.

The original purpose of changing anything at all was to update some of the more dated effects. And let's be honest, the effects in Star Wars are largely great, but some of them just don't fly. If not for the desire to update the bad effects, "Special Edition" would have simply been a film restoration and 5.1 mix and this site wouldn't exist.

The best part is, a lot of those bad effects are still in the films to this day, look on the Blu-ray, you will find them. And contrarily, some of the best effects have since been replaced by worse CGI effects.

No reasoning can be assigned to this, it is for all intents and purposes, random.