The 27-inch iMac has varied storage upgrades varied, depending on which storage configuration is the default. Low and middle models allow customers to choose between 256 GB, 512 GB or 1 TB of SSD storage as upgrades from standard 1 TB Fusion drives, while the high-end model allows upgrading to 512 GB, 1 TB or 2 TB SSD- storage from standard 2 TB Fusion Drive.
However, today's available SSD options on the 27-inch iMac line in Apple's online store are much more limited and somewhat contradictory depending on the boot configuration. On the low-end model, 256 GB is now the only SSD upgrade option available, while Apple in the middle screen model does not show any SSD options at all. For the advanced configuration, only a 512 GB SSD option is available.
Although the change is consistent with Apple's online stores in several countries, the Apple Store continues the iOS app to show all the common SSD options.
The 21-inch iMac is also unaffected in the online store or app. Apple offers up to 512GB or 1TB of SSD storage depending on boot configuration, and it makes little sense for the 21-inch model to offer better SSD alternatives than the larger sibling.
As a result, this is most likely an error with Apple's online store that should be resolved relatively shortly.
Update 4:15 PM : Apple says that this is actually a mistake and is solving the problem.
(Thank you Kenneth!)