One thing I did not realize until a couple of years ago is that original edition D&D used an armor class scale with 9 representing unarmored. I had always thought that was just some Basic edition wonkery, but it is instead really Advanced edition wonkiness.
To make it even more wonkilicious, the Advanced D&D Monster Manual did NOT adjust monsters’ AC at all from the AC 9 Original system to the AC 10 Advanced system. So an AC 6 orc went from three steps better than an unarmored man to four steps better. It seems that they just left monster ACs where they were. Whether this was intentional or an oversight, I do not know. I guess it’s also possible that the decision to go with the AC 10 system had not been made by the time the Monster Manual, the first of the AD&D books published, was finalized.
Comparing Original edition monsters’ ACs to Advanced edition monsters makes it pretty obvious.
Out of 58 monsters listed in Vol 2, excluding men, dragons, giants, and sea monsters (which were either listed “variable” or shared the same AC between types in the original edition), 49 (84%) have the same AC in Original and first Advanced edition despite the change of the base unarmored AC to 10 from 9. That right there should show clearly that no adjustment of monster AC due to the new standard was made.
Of the 9 which have different ACs, only one (the wraith) has the increase of 1 that such an adjustment would be expected to produce.
Below are the changed monsters and what the AC change was in original to first edition:
Wraith AC 3 to 4
Vampire AC 2 to 1
Medusa AC 8 to 5
Chimera AC 4 to 6/5/2
Pixie AC 6 to 5
Dryad AC 5 to 9
Treant AC 2 to 0
Djinn AC 5 to 4
Eftreet AC 3 to 2