That, and some video cards support higher resolutions over DisplayPort. For example, while my 2013 Mac Pro has both HDMI and Thuderbolt/Mini DisplayPort outputs, the only way to get 2160p60 HDMI output (without chroma subsampling) is via a DisplayPort-to-HDMI 2.x adapter (and I had to try about half a dozen different adapters before I found one that does this correctly).