The Mac images are BIOS-only and are for older Macs that don't support EFI. Modern Macs can use the regular PC images, so the name is a little misleading. For example, from the Late-2013 MacBook Pro Ubuntu 13.10 install guide:
"The 2013 MacBook Pro can recognize multi-boot CD image (e.g., Ubuntu's official iso). So there is no need to use the "mac" iso image for installation. In fact, I recommend you avoid the "mac" iso image because it is a BIOS-boot only image and will force the Mac to enter BIOS emulation mode to install Ubuntu, which will need to be fixed to boot in its native EFI mode."
"The 2013 MacBook Pro can recognize multi-boot CD image (e.g., Ubuntu's official iso). So there is no need to use the "mac" iso image for installation. In fact, I recommend you avoid the "mac" iso image because it is a BIOS-boot only image and will force the Mac to enter BIOS emulation mode to install Ubuntu, which will need to be fixed to boot in its native EFI mode."
source: https://help.ubuntu.com/community/MacBookPro11-1/Saucy