Bug or Not? AC and NPC_AC for compendium different for OGL 5e.

Sheet OGL 5e, Mac, LInux and Windows in Firefox. When dragging and dropping a monster it creates to values in the sheet npc_ac and ac. why are these different. Example below of a Lich. I have tried others with the same results. Craven

That is particularly odd. You'll want to use the npc_ac option as this one is correct. I'd have to dig into the 5e code to find why it also populated ac.

I believe this is because the ac  attribute is the calculated ac value for PCs (i.e 10+dexterity_mod if no armor is found in the character's items). A lich has a +3 dexterity mod according to the MM which explains the 13, while the statblock shows them having a natural armor of 17.  I also recall the ac  attribute being one of the few attributes that is always calculated regardless of whether the character is a PC or a NPC.