What is different about the Mac ISO image?
What specifically is the difference between Ubuntu 64 bit for Macs and just Ubuntu 64 bit?
Edit: Is there any difference except UEFI and the boot procedure, like drivers, daemons, etc?
The standard DVD supports both BIOS and UEFI booting, using a multi-catalog where the device is supposed to decide which should be booted.
Although Mac uses EFI (an earlier version of UEFI), it does not support the "multi-catalog", and as a result will not boot.
The Mac DVD uses just the one specific boot image for Mac.