Why is my ice globe lantern flat on top?

IDEA #1: The top of your ice globe lantern is flat because there might have been a lot of air in the balloon before you clipped it shut. If you let the air out, the top of the globe will be rounder.

IDEA #2: The top of your ice globe lantern is flat because the balloon might have had a cap of snow on it while it was freezing. Snow is an insulator and will alter the way the water freezes. Try to wipe the snow away from the top and sides of the balloon when possible.