For weapon tables you are absolutely right, for some ATs, especially AT5 or 6 the criticals start sooner than for AT1 - at least for weapon attacks. For animal attacks, e.g. the Bite Attack Table, this is not true. Here the criticals IIRC start later for AT5 and 6 than for AT1. To me it looks like these armors are not meant for protection in battle but rather as protection from weather. Don't know whether this is realistic. In any case I don't remember having seen a player character actually choose these armor types for his character.
Btw., this topic has been discussed before, so you could try a search for old threads around this.