Why output resolution 1920x1440 not allowed for 4x3 slides?

[billlee]

Relative newbie here.

My slides have 4x3 aspect ratio, so exporting the slideshow at 1920 wide, the height should be 1440. However, when I try to set 1920x1440 as a custom resolution, PhotoStage says "Please enter a height that is no more than 1080."

As PhotoStage export other aspect ratios with more than 1080 in height, this limitation seems arbitrary.

Of course I can use 2560x1440 but this puts wide black bars on the sides.

Of course, it's possible I'm missing some basic insight here.