Yes, the answer to your question lies in your own text. It should be obvious, but apparently it isn't. The FPS in BF4 doesn't go past your monitor's hertz. It's a fairly smart game in that sense. It doesn't stress the GPU more than necessary. Since ULMB requires 120 Hz, that means the FPS cap in BF4 is 120. Thus, what you're experiencing is not strange but entirely predictable. As a sidenote: do you prefer G-Sync or ULMB and why?