Interesting, but I did watch a comparison when BF3 came out of the different settings, and the difference between low to high visually isn't as great as other games. Low settings still looked awfully nice to me, although I didn't actually try them myself. You could install MSI afterburner, and enable the overlay that shows your GPU utilization, and make sure it's at 99% when you compare both settings.
I was actually using Afterburner to find these FPS! I'll play around with it a bit more. I also noticed that visually Low-Ultra was nowhere near as big a difference as most games' relative extremes, probably to ensure everyone playing online has equality. I wouldn't have been shocked if the difference provided by Low-Ultra was only 15FPS or so, but the 2ish (if any) FPS difference I am seeing completely stumps me.
On another (somewhat related) topic:
I run Skyrim on Ultra (No AA) at ~50 FPS in a certain spot.
It runs on Ultra (4x AA) at ~45 FPS. Almost no difference.
Yet on Battlefield 3, enabling ANY (deferred, not post) AA causes a drop of some 20FPS or more. Why the heck is it so much more stressful on the card in BF3 game?