Oh, I know, this is probably why "
they still don't seem right practically-speaking":
The table has the ACs for over 1800 creatures for 5E (mostly WotC by I think it also includes 3PP). The two important columns are By Tier and Cumulative. By Tier is what I showed in my prior post, and considers
only creatures within that tier range (tier 4 includes all CR above 20). However, if you use lower tier creatures in higher tiers (such as an Ogre at CR 2 while in Tier 2), you see the cumulative averages using those lower tier creatures drops the average AC, rather significantly as you get into higher tiers since there are fewer creatures there.
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