There may exist on disk deleted files which are still held open by a program. They can't be removed from disk until they are closed. A common culprit is log files; new admins may try to cull log files by deleting them, and discover that the space is not freed, because the files are still held open by the programs writing to them...
Also note that inodes use space. Space for inodes has to be set aside when formatting the filesystem and is "lost" for file storage. The same is true (although to a smaller extent) for other filesystem structures: superblock, journals (with journaling FSes), ....
To second what bakunin said: with 72M the usage of /dev/xvdb is close to zero and thus negligible, leaving a net capacity of 935G after having lost the space for file system's meta data. Based on that number, the rest of the disks' used space seems to be in a linear relationship.
Aside: All of your disks seem to lose roughly 5% of their capacity to the file systems' data structures.