It isn't an error. If you change any if the bombs by using color burst or saucer, they aren't bombs when they go away. It lowers the total number of bombs.
So, how about you explain it where people understand: some moves change the bombs to regular pieces which takes away from the total you need to finish. So don't use the color bomb with a stripe or it will change the pieces from bombs to stripes and take away some bombs but not count them.