We found a situation where RGBGFX generates more palettes than needed if -Z is used. Here's the image:

Running rgbgfx like so will produce a .pal file with 3 palettes, as expected:
rgbgfx -P gloom.png

However, running with -Z, then 4 palettes are generated instead:
rgbgfx -P -Z gloom.png

Moreover, by changing a pixel in one of the 8x8 squares (so that the square has 4 colours instead of 3), pictured here:

Then running the same command will generate the expected 3 palettes:
rgbgfx -P -Z gloom_fixed.png

The version used here is rgbgfx v1.0.0-9-gd961c697.

(Sprite was done by Cataclyptic)