It was partially relevant in terms of your comment about it being the same as your GS300 - which under the new groups is actually group 37, considerably higher than the CT.
In relation to the difference, it'll be the cost of repairs as a guess. Just quickly looking over the spec, a front end bump in a premier will require the replacement of LED headlamps, which I imagine probably cost quite a bit more than the halogens in the other models.
Again rear end may require replacement of the reverse camera, arguably not that expensive but more than the standard car.
Also can't ignore that the premier costs a considerable amount more than the other two, so in the case of a total loss claim the payout will need to be higher.
Finally I'll add that a 2 group jump on the 50 group scale is not the same as with the old 20 group scale so would hope the difference in actual insurance premiums wouldn't be that large.