This is because until recently, on the UK Store at least, there were two Black Flags you could buy, one was a higher price than the other, but were the same game. A few days ago I noticed one of them was delisted, as on my PS5 it stated you could only "wishlist" it. This is how delistings work as you can see with
Hitman 2. it shows "announced" when you search for it on the PlayStation website.
As you can see
here it states "Not available for purchase" with a release date of 2013.
EDIT: Here's the
link to the old listing, with the PlayStation hits artwork. It errors out, as it's been delisted as compared to the
other listing
Here's an
archive link to the old listing, showing it was for sale up until last November. It has been delisted between Christmas and now. If you look below you'll
see the two versions
Edit 2: Here's an archived
link of the current listing from this month showing both versions were for sale (look below at the 'Editions') and if you go to the exact same link as it is
live on PSN, the 'Editions' has been removed and the PlayStation hits logo one is no longer for sale.
TLDR: The game showing up as "announced" is the now delisted PlayStation Hits £15.99 version which was delisted in the last couple days between March 13th, and today, March 28th. The £24.99 version has taken its place, and reduced in price to £15.99. It's just a cleanup of the listing.