How memory is allocated to containers in the host os

REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
Image_Name         latest              d8dcd701981e      About an hour ago        6.565 GB

I am trying to run this image in my host OS(ubuntu14.04) which is running using a virtual box. I have assigned 4GB RAM to my host Operating system but when i am running my container the memory limit being shown is 5.028GB.
How is this possible??

CONTAINER           CPU %               MEM USAGE / LIMIT     MEM %               NET I/O               BLOCK I/O
gigantic_cori       0.02%               101.5 MB / 5.028 GB   2.02%               44.43 MB / 20.51 MB   580.2 MB / 3.196 GB