How strange!
No one has pointed out that the price has changed because, between the date of submission and availability, was released the iMac 5K, which comes with GPU R9 M290X, or M295X, and LG panel with 5120x2880 pixels priced from $ 2499?
Being an All-In-One the iMac does not need to OUT ports and protocols "late". It runs in the 5K, with 30 bpp at 60 Hz, although it is not yet available Dysplay Port 1.3. Great invention the one Apple!
Since this article Tom'sHardware I would expect to be informed if the panel in the Dell UP2715K is the same LG Apple iMac or if it is different.
In the Monitor Panel Part Database:
http://www.tftcentral.co.uk/articles/monitor_panel_parts.htm
I find only
LG.Display - AH-IPS - LM270QQ1-SDA2 - 5120 x 2880 - 14ms G2G - 1000: 1
Sharp - IGZO LQ270D1JG01 - 5120x2880 - ??? - 1500: 1
... and all these technical look a little different from the ones provided by Dell.