I get you. The purple circles are not kissing other circles, but the yellow ones -- not sure. In general it seems to have to do with how constrained they are.

I get you. The purple circles are not kissing other circles, but the yellow ones -- not sure. In general it seems to have to do with how constrained they are. And there may be differently-arranged also-optimal packings - e.g. for N = 18 there are others besides what is shown.