Unfortunately, while there is some correlation between the armour value and the armour the soldier is actually wearing (eg, higher armour values appear to be wearing more armour) there are lots of inconsistencies, even down to that some soldiers who have shields get no shield bonus, while others who have no shields do get a shield bonus.
Check over on the Ludus Magna forum with the game researchers, this is more their bag. There's a thread on inconsistencies in the unit file, which goes through some of the more glaring armour examples.
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