He means the defense... but i think it's a feature rather than a bug.
Usually you can upgrade to full plate only if you already have partial plate (it wouldn't make sense to upgrade archers or other 'fast' non-armoured untis to heavy armor), right? There's no big difference between partial and full plate, like there is no big difference between light and heavy mail, hence the same increment of only 1 point. But some units like militia pikemen that starting with no armor can upgrade all the way up to partial plate get +3 armor...
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