Can't match program size to screen

[jonnygexter]

I can't seem to match my base resolution to my full iMac screen, I get black bars around the extra space in the recorded video and in the program display. My iMac's display resolution is set to 2560x1440, which matches my base resolution and output resolution in OBS. What am I doing wrong?

I can tweak the numbers to remove the bars (~2400x1350), but this seems strange...