because of wow classic gold add-ons and mechanical leveling?
There were plenty of cases in vanilla of comparable abilities stacking, and at times folks exploited that. It is possible that the foundation way resistance and armor function is incorrect on personal servers, which would also account for a great deal of differences. But my guess is that every armor reducing capability will stack with each other. In general in WoW Classic, comparable effects piled, but the exact same skill didn't (that is why only one battle yell could be busy, and that means you can not just spam warriors to all sunder armor and trash the boss).
This is something I'd expect. Just Blizzard had the original data. Personal servers use server code predicated on what was reverse engineered and since virtually all of the worthwhile world information (describe mechnaics, loot tables, experience prices, AI, pathing, dialog scripting, etc.) had to essentially be recreated from scratch. Oftentimes they did get things wrong due to incorrect understanding or improper assumptions. In other instances things were intentionally altered like experience prices and rear porting new races from newer DLC.One of the things which I do hope they do is give us appropriate widescreen support. Widescreen support was clipped incorrectly giving widescreen monitor users a dis-advantage by cutting the top and bottom of the display instead of wow classic gold raising the FoV for your participant. This was fixed via a 3rd party patch and afterwards patched properly but 1.12.2 foundation has this matter.