Partition GUID Code vs Partition Unique GUID – Differences

Found the answer: the Fedora ISO contains a GUID Partition Table with a partition layout very similar to that of OS X. Because of this, I confused the partitions created by

dd if=Fedora.iso of=/dev/sdb

with the ones created by the OS X installer. The confusion was furthered by the fact that one of the partitions has a HFS+ filesystem, which is specific to OS X. What's even more curious is the fact that running parted after writing the ISO to the stick yields:

$ sudo parted /dev/sdb
GNU Parted 2.3
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            
Warning: /dev/sdb contains GPT signatures, indicating that it has a GPT table.  However, it does not have a valid fake msdos partition table, as it should.  Perhaps it was corrupted --
possibly by a program that doesn't understand GPT partition tables.  Or perhaps you deleted the GPT table, and are now using an msdos partition table.  Is this a GPT partition table?
Yes/No?

Anyway, the point is that the partitions were not magically reinstated after zeroing the entire device, instead they were created when dd-ing the ISO.

Partitioners like to align partitions on a mebibyte boundary these days. For MBR partitioning, there are 4 primary partitions, and for the rest you need extended and logical partitions.

While the layout of the primary partitions is expressed at the end of the first sector of the disk, for the logical partitions, you've got a linked list of additional partition tables (themselves specifying only one partitition. Typically, the first one is as the beginning of the extended partition (which is itself defined as a primary partition) and defines the first logical partition, and it links to the next partition table which defines the next logical partition. That next partition table will be located typically after the first logical partition.

All those partition tables only take a few bytes outside of the partitions, but because of the mebibyte alignment, a full mebibyte has to be used for them.

GPT on the other end stores all the partitioning information at the beginning of the disk (with a backup at the end), so after converting, that space that was used for the logical partition partition tables becomes free.

Note that you only need one sector to store those MBR logical partition tables, so strictly speaking, in MBR partitioning there would be 2047 sectors free, if the partionner was willing not to align partitions on mebibyte boundaries.