The full_armor tag explicitly states there needs to be covering for the body, arms, legs, AND head. Despite this, people keep adding the tag for pictures where none of the characters are actually covering their head.
Because drawing a character's face is generally very important in illustration, it's quite common for them to have an uncovered head even when they logically should be wearing a helmet, even when they are otherwise fully decked out in armor. It happens so frequently that it deserves its own tag.
On a related note, people REALLY misuse the helm tag: it refers to a single piece helmet that covers the entire head, yet people often include helmets with hinged faceplates or no face coverage at all. Just looking at the 20 images on the first page, at least 11 of them are wrong, and some of the others are debatable.
Updated by Rampardos