Additionally, sales of EA's Battlefield 4 are down 69% compared to its predecessor Battlefield 3.
Both games - which are out now for current-gen platforms - have apparently underperformed at retail due to the forthcoming release of two "must-have consoles", Xbox One and PlayStation 4.
The next-gen versions of the titles are said to be the definitive editions, thanks in part to better graphics and updated controllers.
"We've never had two must-have consoles launched so close together," GfK Chart-Track director Dorian Bloch told MCV.
"The unquantifiable question is, how many consumers are waiting on these franchises for a few more weeks?"
Bloch added: "How many PS4s and Xbox Ones will have sold by Christmas? I would say, let the dust settle on 2013 and then we will know more."
Similarly, Call of Duty: Ghosts has failed to outsell its predecessor at launch reportedly due to it coming to Xbox One and PS4.
The next-gen versions of Assassin's Creed 4: Black Flag, Battlefield 4 and Call of Duty: Ghosts will be available when the new systems arrive later this month.
The Xbox One launches on November 22, while the PS4 will hit the UK on November 29.
Watch the launch trailer for Assassin's Creed 4: Black Flag below.