We also know that the '90s dubbers didn't have much to work with when creating the dubbing script:
Craig Allen wrote:All the English script-writer gets is a translation of the Japanese; there's no briefing on the background or history of the stories, or anything like that. So sometimes he doesn't fully comprehend what's supposed to be happening, and sometimes we have to make last-minute changes to the script in the studio...I know we sometimes get the details wrong.
Anyway, I assume the translation situation was the case in the '60s and '70s as well. That would explain why the ToMG dub uses "Radon," just as both versions of Godzilla vs. Mechagodzilla II did.They often gets nearly unintelligable English translations to work with. They then have to write the English to fit the lips of the on-screen characters. (And they usually have to do *very quickly*.)
One thing Allen didn't mention is that either the scripter or the original translator has to make the dialogue sound like English (instead of a literal translation of Japanese), and that would include inserting pronouns where the Japanese dialogue doesn't use any.