Reach is calculated from the players head so when you are above someone you effectively have 1 block less reach then them, so when you are jetpacking and a player on ground melee's you when you cannot melee them its not reach they hit your feet. I'm including a shitty paint diagram to help show what is happening
Hitboxes also always have the same orientation meaning that you can hit people from farther away if you come from the corner of the block. Be careful tho, its also easier to get combo'd.