Dishwasher safety - "top shelf" vs. "bottom shelf"
In most dishwashers the heating elements are on the bottom only. Lightweight plastic items might melt (soften and distort, not drip away) on the bottom, but they don't mind being in the steamy environment or being sprayed with hot detergent-laced water. They are therefore only dishwasher safe on the top shelf.

I've melted a plastic bowl or two in my time; there is a definite difference between top and bottom.