Do you have a digital caliper?
.2mm is not enough difference. 31.8 is the handlebar at the center and the threadless stem on the headset. You can use another tool if you do not have a digital caliper to compare the center of the HB to the existing seatpost. They should be not quite exactly the same by 0.2mm. If so it is a type-o and order the 31.6mm dropper post.