That is the D&D 5e 2024 by Roll20 sheet. Regarding npc_ac, it probably is due to backwards compatibility. My guess is that much of the 2024 sheet was born out of the logic of the 2014 sheet. Additionally, people have macros, API Scripts, etc that will be looking for npc_ac. Finally, of course npc_ac does not factor in the NPCs Dexterity. It would break things if it did. All of those 2014 creatures do not use Dexterity in their AC calculations. Why? Because WotC decided not to use calculations for NPCs. They "gave them whatever stats they wanted which may or may not line up with their ability scores".