SSD total capacity <> free space + used space

Question

I got a new 500GB SSD and formatted it with ext4+LUKS using the Disks utility in Ubuntu 16.04.

1. Enter disk with Nautilus, right-click and Properties. Total capacity is 492 GB, free space is 467 GB, and used space is 73 MB. Shouldn't Total = free + used?
2. Using df -H <mydisk> I get the same values as nautilus. Using df -h <mydisk> I get total size 459GB, available 435GB, and used 70M. (Notice -h is powers of 1024, and -H is powers of 1000.)
   • 500 * 10^9 / 2^30 = 465.66 GB meaning the total size of 459GB makes sense when using powers of 1024. However, the discrepancy between total size and available+used space is still present.

Total size of the disk is quite different from the sum of the free and used amounts. Why? answer

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This is a secondary disk. The OS is installed in a different disk.

$ sudo tune2fs -l /dev/sdb 
tune2fs 1.42.13 (17-May-2015)
tune2fs: Bad magic number in super-block while trying to open /dev/sdb
Couldn't find valid filesystem superblock.

$ lsblk /dev/sdb
NAME                                        MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sdb                                           8:16   0 465,8G  0 disk  
└─luks-<long-random-string-1> 253:3    0 465,8G  0 crypt /media/daniel/<long-random-string-2>

$ sudo tune2fs -l /dev/mapper/luks-<long-random-string-1>
<this works>

How do I recover the reserved space? (this doesn't work)

Edit: I've reduced the reserved space using $ sudo tune2fs -m 0.3 /dev/mapper/luks-<long-random-string-1>. The problem was that I wasn't using the correct device (drive is encrypted).