Certain sprites are hard-coded to use the sprites out of a certain box. Here's a list of which spritesets use which one:
Box #1: Cemet, Cent, Eggs1, Eggs2, Guest, Hell, Island, Kings, Maze, Moon, Plant, Priest, Ravil, Red, Sand, Stream, Toro, Weed.
Box #2: Almo1, Almo2, Ballos, Bllg, Curly...