I don't know the whole story, but seeming as this was banking software I wouldn't be surprised if it was indeed written some time in the 80's or 90's and the original developers were nowhere to be found.
i know it's not your first language but since you mentioned you like to think and understand things, the correct syntax is '80s and '90s; the apostrophe is used to indicate letter omission. incidentally, my first language was basic. didn't want to omit that :)