Hiya Sorina,
When you use the custom date range, Google isn't listing results based on the date they were indexed. Google is using an estimated publication date.
Google tries to estimate the the publication date based on meta-data and other features of the page such as dates in the content, title and URL. The date Google first indexed the page is just one of the things that Google can use to estimate the publication date.
I also suspect that dates in any sitemap.xml files will also be taken into consideration.
But, given that even Google can't guarantee that it'll crawl and index articles on the day they've been published the crawl data may not be an accurate estimate.
Also, if the scraped content is being re-published with intact internal links (are these the full URL - do you they resolve to your original website?) then it's pretty obvious where the content came from.
Hope this help answer your question.