Little question about Defense rating

[tinkthank]

This has been bugging me more while trying to figure out optimal results for modding:

A unit will have a "basic" defense stat. This can be changed in-game by any number of effects. Most notably: Armor, Weapons, experience, wounds, and various spell effects.

But this rarely adds up correctly, even in the "recruitment" screen, where no experience or spell effects can factor in.

Example: A standard, unmodded Van has in the Recruitment screen (thus before he can have any experience or effects cast upon him) a defense value of 18; his "basic" value is 14, and his items yields a net defense bonus of +1 (+2 for shield, -1 for cuirass). Thus he has 3 more defense points than is justified.
This phenomenon can be observed with many units (so it is not just anything wacky with Glamour).

Any canonical justifications?