Generally speaking, EQ is not a volume control.
With originally reserved gain, EQ can rise volume for selected frequencies which is the case for most home cinemas. For limited resources of phone, it's unpraxtical to reserve gain, so volume of zero is actually maximum possible volume. Rising volume over this point will produce distortion (or will produce sound louder than allowed by some countries). That's why this EQ lowering volume of non-selected bands. EQ is doing what is supposed to do - sound equalization. What it can't do - is to rise selected band above 0 (which is what you're expecting).
Samsung Galaxy S4 SHV-E330S: Optimus ROM (TW 4.4.2) & Kernel. CPU OC 2.7GHz, GPU OC 645MHz, non-KNOX bootloader, UHD/SM/360 Camera.
Samsung Galaxy S4 GT-I9500: Optimus ROM (TW 4.3), Perseus Kernel. CPU No-OC, GPU OC 700MHz, non-KNOX bootloader.
Past models: Samsung Galaxy SIII SHV-E210K & GT-I9300, Samsung Galaxy SII GT-I9100.