incorrect RAM being displayed when i press F3

ok so 1024MB = 1GB
in the profile editor on the mine craft launcher, and theres JVM arguments
which was -Xmx1G im assuming 1G is 1GB of ram
however when i go to play mine craft and i press F3 it says for example 120mb of 910MB, and 1GB is not equal to 910mb
i changed it from 1G to 2G(2GB) and it said 1820mb
i also changed the G to M so now its measured from the profile editor in MB i put in 1024mb(1GB) and it still said 910mb in the game i also then changed this to 2048mb(2GB) and it was 1820mb