Which are they? It's important to include links so that (a) we can say what's going on, and (b) so readers can make up their own minds.
You might be talking about stories that went in to the second-chance pool (https://news.ycombinator.com/pool, explained at https://news.ycombinator.com/item?id=26998308), which get a random placement on HN's front page.
I have heard that sometimes the submission time is reset, such as when returning from the second chance pool. This could also create an unnatural ranking order, so the original time before the reset should be listed as well.
Yes, the timestamp munging is an artifact of HN's re-upping system, described at https://news.ycombinator.com/item?id=26998308 and links back from there. About the timestamps, there are past explanations here: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que....
The original times are always available because the modified time is used only on the frontpage and the article's /item page. If you find the article on /from or /submitted, for example, the timestamp will be the original. The two timestamps converge over time.