I noted that the old duplicates and the new duplicates have different formatting.
The questions that are now getting closed as duplicate get a bluish plaque on top saying "This question already has an answer here: ..." But the questions that were closed as duplicate long time ago have a grey plaque on top saying "Possible duplicate: ..."
If you look at all the duplicates sorted by date, then the borderline between these two formats seems to pass around January 2013, currently page 19. On page 20 duplicates are already mostly of the old format (even though it seems to depend on the closing date, not on the posting date).
The old format looks quite obviously inferior: it has strange wording ("possible"?), does not display the number of answers in the linked question, and does not update the title of the linked question if it gets edited (the link seems to be "frozen").
I searched the meta.stackexchange, but could not find any reference to this behaviour. I assume that the format was changed at certain moment, but why are the old duplicates not displayed in the new format? This seems weird. Any explanations?
Here is one of the old duplicates:
If you start editing this post, you will see that it starts with the following text:
> **Possible Duplicate:** > [Replacement of NA values for PCA analysis](https://stats.stackexchange.com/questions/35561/replacement-of-na-values-for-pca-analysis) <!-- End of automatically inserted text -->
so the link to the dup is actually hard-coded into the question text. Well, I thought if I edit this question (and remove this automatically inserted plaque), it will trigger automatic re-generating of the HTML page and it will get the "new" blue duplicate plaque. This did not happen! I was able to erase the old hard-coded plaque, but the new one did not appear. The post was marked as duplicate without any link to the dup. I edited again and inserted the hard-coded plaque back.
This is weird.