New monitor and resolution won't stretch?

I bought a new monitor today, a BX2331 Samsung, and on the characteristics it says that the best resolution for it is 1920 by 1080, I've tried it and the problem is on left-right and up-down alot of the screen is black as if the resolution is too small for the screen but the thing is that the writing and everything are way too small as if the screen was too small for that resolution, I've putted 1920 by 1008 and on the left-right side it fits perfectly however up-down there are still black spaces so logically the 1080 one with 70 more pixels should fit the up-down perfectly, so I just don't get it why instead of filling the screen it makes it self smaller, any ideas?