If I'm picturing this right, it probably has to do with the shape of the toys.
The diameter is a cross-section across the widest part, while the circumference goes around the toy's perimeter. So say you had a toy that, when viewed from the top down, had a slightly more oval shape. The diameter would be greater, but it might still have the circumference of a similarly-sized, perfectly round toy.
I think that's how it works, anyway. Not sure if that's the case with these particular toys, though.