Because the R9 380 is wholly incapable of running Battlefield 4 at UHD. Even at half that resolution, the 380 can only manage 30FPS, so half the frames at double the res is quite appropriate.
And at UHD even the 290X, which is near 40% faster than the 380, can only manage to average 37fps. Battlefield 4 Performance - 4K UltraHD (3,840 x 2,160) | bit-tech.net
The only card that can single handle 4k is GTX 1080, until now people made multi-gpu profiles for 4k