Yeah, that's just how Assassin's Creed Brotherhood performs. I have a 6870, a little bit slower than the 560Ti and I get similar performance at 1080p, sparser areas the game will run 60FPS no problem, but the framerate will drop into the 40s when things get busy on screen. Ubisoft tends not to put a huge amount of effort into optimizing the PC versions of a lot of their games, so that probably doesn't help. Looks like you are getting the performance you should with your hardware.
The biggest problems with BF3 is the fact that it utilizes a CRAP TON of vram even at its lowest settings. At Ultra settings, (2xmsaa) it requires like 1.25 gb vram, so a single 560ti will be very badly bottlenecked in that sense.
Otherwise though, it seems that 560ti is working just fine.