Hooked up an external monitor to a Thinkpad through HDMI, maximum resolution available is 1920 x 1200 when the monitor's native resolution is 2560 x 1600.
Manual says:
4096 x 2304 pixels / 60 Hz, with an external display connected to the USB-C connector
4096 x 2160 pixels / 24 Hz or 3840 x 2160 pixels / 30 Hz, with an external display connected to the HDMI connector
It seems Thunderbolt is the preferred option, but I have a HDMI cable and would like to use it, is the resolution limited by the cable?
Are you trying to Duplicate the display? As that can cause it to find a resolution compatible with both displays. Try outputting to the external display only. Or Extended desktop.