I don't have SW available right now and probably couldn't open your part if I did, as I am still running SW2010, so this is just a guess. I suspect that on the ladle you did not close the center face. When you mirror it and knit, it no longer needs the center face to make a water-tight surface and is able to make a solid.
I suspect you made the handle with the center face. It makes a water-tight surface before the mirror and, hence, can make a solid. You should not even be able to knit the two bodies after you mirror, unless you picked just the "outer" faces, not the whole body, to mirror.