By default, ext2 and its successors reserve 5% of the capacity of each filesystem for use by the root user. This reduces fragmentation, and makes it less likely that the administator or any root-owned daemons will be left with no space to work in. Whether these considerations justify the loss of capacity depends on what the filesystem is used for.
Reserved space is least useful on large filesystems with static content that are not critical to the basic functionality of the operating system. In such cases it is quite reasonable to reduce the reservation to 1 (or 2) percent.
To do so, use the tune2fs system tool with the option -m to specify the new reserved % value (no need to unmount file system):
# tune2fs -m 1 /dev/sda1