Oh, I was just looking at the updated PDF and I saw that all the sample monsters' evasions scores were updated with the errata. It makes more sense now!
Ability mods don’t come into play when defending, but if a creature has any amount of DR (from items or effects, etc.) they gain a +X to the check to defend, where X is the DR.
So for example, if a monster’s attack is 3, normally a character has a 3-in-6 chance at evading the attack. But if they’re wearing DR 1 armor, that becomes a 4-in-6 chance at evading the attack.
As an aside, I’ve been working on a few house rules regarding morale, sundering armor, and some other stuff I was planning on doing an errata for in the coming weeks, so keep an eye out for that!