In 1.9 or below, your spawn radius is 10, meaning you would spawn in a 20 by 20 square by default. In the 16w20a you spawn in a 21 by 21 square by default.
MC-102245
Spawn area in snapshot 16w20a is 21x21 instead of 20x20 square
Resolved
Works As Intended
0
The spawn point is a single block, 10 blocks north + 10 blocks south of that + that block = 21.