Publication estimated time
"Estimated publication date" - we have not set a fixed date for publishing this article, but this is a guess based on how long we think it's going to take.

"Due publication date" - we have set a fixed date for publishing this article.

In this case, I'd guess that they haven't yet reviewed the article (so can't be sure they're going to publish it) but know from experience that it usually takes about eight weeks to review + publish. When it's been reviewed, they'll be able to set a due date.