yeah, my guess is that it happened in the rendering. while you chose to render the file to 24 bit, perhaps you had the UV22HR (or whatever other dither you used, but if you're using WaveLab, my guess is that you're using UV22HR) set to 16 bit. that would cause the file to be dithered to a 16 bit file, yet stored in a 24 bit file. but it's hard to say exactly what happened.