Why is the 2012 imac firmware update failing on 27" imacs?

Because apparently your machine does not need it. You can call AppleCare if you have additional questions about the firmware update.

Still unknow but fairly widespread. From MacRumors Forum:

"Spent about half an hour with Apple support after both of today's updates failed on my 27", 2012 iMac, telling me that neither was supported by the machine. They were surprised at the updates not working and downloaded info from my machine to forward on to engineering. We're scheduled to talk again on Wednesday to see what they've found out about this problem. I told them that there were others besides me experiencing this issue."

I have read a couple dozen posts about this issue, think it will be sorted out soon. I have the same issue.

I have seen that suggested in other posts but all documentation on the Apple website says ALL late model 2012 iMacs and the posted coversation from Apple Support suggests otherwise. Personally I think it's a technical glitch that will be resolved shortly.

Maybe as the 27 inch models were built quite a bit later than the 21.5 inch models they already have the firmware applied?

Neither will work on my new 27 inch iMac either, which arrived yesterday.

The link you posted:

http://support.apple.com/kb/DL1618

Actually says:

System Requirements

• Mac OS X 10.8.1 and greater, EFI version 010A
• iMac (21.5-inch, Late 2012)

Copied from this link today:

http://support.apple.com/kb/DL1618

About iMac EFI Update 2.0

This update is recommended for the iMac (21.5-inch, Late 2012).

This update contains general performance fixes pertaining to sleep and Thunderbolt, and improves compatibility when using the 5GHz band in Wi-Fi.
Boot ROM or SMC Version Information: After Update is complete:

Build version: 010A.05
ROM version: IM131.88Z.010A.B05.1211151146

And here is a screen shot of the link as well:

Very clear that it says 21.5 inch model.