Free space is calculated by rsp probe by "df" command, simply we subtract from (size_mb - used_mb) then we get free space.
"df" command tells you the amount of free space on all mounted file system or specified file system. In "df" command file system reserve some percentage of memory for the root user, so it is showing less available space under "Available" section.
There is some difference between Available and free memory.
free memory = Total memory - used memory.
Available Memory = Total memory - used memory - reserve percentage of memory(for root user).