I think you made everything in correct. So, probably your iso image contains a GPT partition which seen on after writing with dd.
You can check specific filesystem or partition table signatures in a device with wipefs command:
$ sudo wipefs /dev/sdb
offset type
--------------------------------------------------
0x0 mac [partition table]
0x200 gpt [partition table]
0x8001 iso9660 [filesystem]
LABEL: PVE
UUID: 2015-10-06-08-19-28-00
You can remove all of the signatures from the device (with the help of the -a parameter) before writing new iso image with dd. After iso image write process, you can check signatures again and compare with previous ones.
See also: man wipefs