I don't know the official difference, but I think for this site, it is based on how the credits appear on screen. Some people are credited with "producer" and others with "produced by". Sometimes it even happens within the same production.
Just piggybacking onto Adrian's response, this is correct. The job type is essentially the same thing, the only difference is how the credit was displayed on-screen, this is also common for Writer credits (ie. "Writer", "Written by", "Creator", "Created by", etc.).
