How are armor class values calculated for monsters in terms of ordering them?

[nathaniellee]

I noticed that when ordering monsters by armor class, the Dryad, the Druid, the Archmage, and the Mage were all considered to have the highest values. They each have fairly low -- 11 or 12 -- "default" ACs and then some sort of bump for a second value.

It seems like in each of these cases, the total value of the two combined, however, is 27, which would put them above the highest solo AC which is the Tarrasque's 25.

However, there are three other monsters that also have multiple AC values: the Ankheg, the Azer, and the Lizardfolk. And in fact the Azer and the Lizardfolk's combined AC values would be even higher at 32 and 28, respectively.

The only thing I can suss out here is that perhaps it's correctly ignoring the second value when that value comes from a condition or armor, but combining the value when it comes from a spell?