Comment 3 for bug 1720885

The reason is luks-volume contains LUKS header in the device, it costs 2M (512bytes * 4096sectors).

[root@localhost ~]# cryptsetup status /dev/mapper/crypt-scsi-36001405eb821aa8e66e44ecbfec83dfa
/dev/mapper/crypt-scsi-36001405eb821aa8e66e44ecbfec83dfa is active and is in use.
  type: LUKS1
  cipher: aes-xts-plain64
  keysize: 512 bits
  key location: dm-crypt
  device: /dev/sde
  sector size: 512
  offset: 4096 sectors
  size: 2093056 sectors
  mode: read/write

[root@localhost ~]# cryptsetup luksDump /dev/sde
LUKS header information for /dev/sde

Version: 1
Cipher name: aes
Cipher mode: xts-plain64
Hash spec: sha256
Payload offset: 4096
MK bits: 512
MK digest: 5d 6a d7 86 41 d0 f4 0a dd 55 c7 c6 ff f5 ed 63 76 e9 61 fd
MK salt: 77 c2 88 b6 a0 7e cb 18 f2 79 63 09 19 14 dc 76
                40 8b 0f d5 eb aa 0d ef ba 85 e3 58 75 48 bc df
MK iterations: 15784
UUID: 83431299-e4c5-4999-b007-973425725ace

Key Slot 0: ENABLED
        Iterations: 279470
        Salt: 8e 57 e5 c6 d2 65 d2 da 94 bb cb a7 65 e3 2c 6d
                                d1 46 7d 30 0e 1f d1 33 89 39 f9 97 30 54 aa 17
        Key material offset: 8
        AF stripes: 4000
Key Slot 1: DISABLED
Key Slot 2: DISABLED
Key Slot 3: DISABLED
Key Slot 4: DISABLED
Key Slot 5: DISABLED
Key Slot 6: DISABLED
Key Slot 7: DISABLED