Ext4 as default allocates 5% for superuser use only. This is to make sure that the drive doesn't get filled completely, which would make reallocation of data and other operations fail. This amount can be lowered (often very OK today with the giant drives we have) or removed (not recommended on e.g. /home and system partitions, but on pure storage drives, the negative effects are usually not that large. Perhaps increased fragmentation as the drive fills up to the very rim).
With that out of the way, there is also space reserved for "inodes".
A typical allocation heuristic for inodes in a file system is one percent of total size.
which might very well be a good rule of thumb (in practice I'd say it's more of 1.5-2%, but the order of size is correct).
On NTFS the "inode" concept is reflected as the MFT and file ID:s. The MFT grows as more files are added, and are thus not fixed size, and will therefore show small overhead on an empty file system.
And lastly, there is always the confusion between binary prefixes and reported size by drive manufacturers.
See http://askubuntu.com/questions/48488/ext4-partition-size-free-space-discrepancies/48511 for another similar question which touches most pitfalls.