2012 Mac Mini not displaying in 1920x1200 on a 27" IPS Display
The Mac Mini can do 1920x1200 but it is probably the HDMI to DVI adapter doing the limiting.  The Mac will only display the resolutions that it is being told it supports by the connected device.

If you are trying to get the display to 1080p HDTV, it is doing it properly (HDTV is 1920x1080).

If you are trying to get to 1920x1200 for a different monitor with a DVI connector, it probably won't be able to, again due to the limit of the adapter.

The monitor can probably do the 1920x1200 but it can't communicate that to the Mac because the adapter is in the way, which is saying it can only do 1920x1080.

By chance does your display have other connection options, like DisplayPort or MiniDisplayPort?  You can get an HDMI to Display Port adapter which will allow the display and Mac to communicate properly.