3840x2160 monitor. Can't go higher than 2560x1440.

Macbook Pro, OS X 10.10.5

Nvidia GeForce GT 650M.

Samsung U28D590

When I hook up this monitor using an HDMI cable, I can get up to 3840x2160 @ 30Hz. When I switch to a DisplayPort 1.2, it caps at 2560x1440 @ 60Hz.

Am I capped at 2560x1440? When I was on the HDMI, was I actually running at 3840x2160 or was it running in a "looks like 3840x2160" state?