Based on various reports at the time of the bluray re-release, it sounds as if the first restoration was a photo chemical (film) restoration from YCM separation elements, while the last release was from the camera neg, now made possible via digital restoration techniques that are not possible when doing a 100% analog photochemical restoration.
This thread has interesting info:
https://www.hometheaterforum.com/community/threads/a-peek-at-journey-to-the-center-of-the-earth-redux.339467/page-2