Spigot drastically altered vanilla mob spawn logic and caused a few issues.
1) Used only spawnable chunks vs entire world for entity counting, resulting in ignoring
other entities in the world, and causing the world to go over its intended limit.
Specially with servers using smaller mob spawn ranges than view distance, as well as affects spawning API
2) Spigot was using 16x16 division instead of vanilla 17x17 division.
Issues got worse in 1.9 due to more chunks being loaded due to 1.9 changes, that fall out
of the monster spawn radius.
This patch returns mob counting to use all loaded chunks, and 17x17 division.
Aikar