Pre-orders and previews begin April 10, launches April 24.
'Mid 2014' 27-Inch iMac Also Referenced on Apple Support Page
Apple did release a new low-end Mid 2014 21.5-inch iMac last month, but the remainder of the iMac lineup remained untouched and those machines are still considered Late 2013 models.
While the Mac mini entry has been taken as a hint that an upgrade may be imminent, especially since the machine has not been updated since October 2012, the iMac entry may point more towards the inclusions being an error. With the new low-end iMac having been added just last month, it seems unlikely that further iMac updates will occur in the near future.
It is possible Apple may have planned for a broader iMac update in the middle of the year with the low-end model appearing as part of that refresh, but the company may have changed plans due to a lack of chip availability or other factors. Alternatively, it may simply be an error unrelated to Apple's product launch plans.