Defence Skill?

[Hully]

I'm currently playing a game as Man Last of The Tuatha. Most of my special units have two more defence than I can account for?

Example: Arch Druid

Basic Def 11
Exp +1
Current Def 15

Example: Sidhe Champion

Basic Def 15
Exp +3
Current Def 20

And just for interest

Example: Monk

Basic Def 8
Exp +1
Current Def 8

Just a little confused, all the standard unit are ok

Pretender +4 fire +4 Nature at start.

So... I'm missing the obvious right?