I have noticed, throughout my years of playing Minecraft, that when a splash potion lands, it creates those swirly particles offset from where it actually lands. If you throw the potion in the center of the block, it will appear in the corner. Always. No matter where you throw it, the particles of the potion will always appear in the corner of the block. If you throw it on the wall, it will still appear on the bottom side of the block. It always spawns the particles at (0,0,0) relative to the block. So if the splash potion lands at (0.6,0.2,0.9) block-relative, it will still create the particles at (0,0,0) block-relative.