Can't access 2560 x 1080 display resolution settings on MBP I've got a mid 2012 15" MBP running 10.10.5.

I've also got access to a mid 2012 13" MBP running 10.12.6.

I'm trying to connect both (separately one at a time for testing) to this LG ultra wide display which is a 21:9, 2560*1080. 

I chose this monitor as its the widest these 2 MBPs can support according to their tech docs here: 

15": https://support.apple.com/kb/sp694?locale=en_US
13": https://support.apple.com/kb/sp649?locale=en_US

Both of those docs state:


  Dual display and video mirroring: Simultaneously supports full native
  resolution on the built-in display and up to 2560 by 1600 pixels on an
  external display, both at millions of colors


When I connect the 15" MBP to the display it works fine and I can select 2560 x 1080. (see screenshot below)



But when I connect the 13" MBP the biggest size is can set is 1280 x 800, even if I 'option key' click the scaled button. (see screenshot below) Any ideas why I can't access the full set of screen dimensions on the 13" MBP ?
I got this working by using a mini display port (MBP end) to display port (monitor end).

Not sure why it worked on the 15” with the hdmi lead but not on the 13” as it was the same 4K compatible hdmi lead.