Simultaneously, Apple also released a supplemental update for OS X 10.8.5.
iMac SMC Firmware Update 1.1
This update is recommended for iMac (late 2013) with flash-only storage. This update improves the Power Nap feature in Mavericks on your flash based iMac by allowing it to perform silently, without spinning up the fan.
Power Nap periodically updates apps, such as Mail, Contacts, and Calendar, and will download software updates while your iMac is sleeping.
iMac 10.8.5 Supplemental Update 1.0
The iMac OS X v10.8.5 Supplemental Update 1.0 is recommended for iMacs (Late 2013) using NVIDIA GeForce GT 750M graphics and running OS X Mountain Lion v10.8.5.
- Fixes an issue that may cause external drives to be ejected after the computer goes to sleep
- Fixes an issue that may prevent certain USB Bluetooth adapters from working
They aren't the first updates for the new iMacs -- when they were first released, Apple released an EFI update to address Boot Camp installation issues.