Sorry for the confusion. It's because when moderators or story reviewers re-up a post, as described at https://news.ycombinator.com/item?id=11662380, the timestamp is adjusted to be the re-up time, on the front page and the comments page (/item).
Well, not precisely the re-up time—rather the timestamp that answers the question, "when would a submission with this many points have been posted in order to have this rank on the front page". Of the three variables—points, timestamp, rank— rank is normally the dependent one, but on re-upping, timestamp becomes a dependent variable since the software is setting a rank (randomly, but usually around #20).
If you see a timestamp inconsistency on HN, this is likely why. You can always tell which post is the earlier one by comparing the IDs in their URLs. The timestamps converge after a day or two, so the confusion is at least temporary.
The original timestamp remains visible on other pages, such as /from?site=domain.com and /submitted?id=username, e.g. https://news.ycombinator.com/submitted?id=chkuendig and https://news.ycombinator.com/from?site=dailydot.com for the current article.
We started doing this because it turned out that if you lob a story onto the front page with an old (say "2 days ago") timestamp, the thread will turn into "how is this story on the front page when it was posted 2 days ago".
If the thread has comments, the timestamps on those comments also get relativized. (Otherwise the thread would fill up with "how can this comment have been posted before the submission?!!")