Converting image to indexed via BIMP produces different results than via the GUI

[MrEveryDay98]

On the left is the original image. In the middle is the image converted to indexed (2 color maximum) via the GUI. On the right is the image converted to indexed (2 color maximum) but via BIMP.

[kcbehler]

That's looking like the GUI is wrong and batch is correct. Two colors means black and white. Gray is a third color. Am I wrong?