I detect spinning top tank syndrome to be the cause. As a tank it is relatively easy to avoid changing a mobs position during an AOE. The only time you might run into this is with massive trash pulls where the tank needs to move out of multiple AOEs that are somewhat overlapping. But in that case hopefully a BLM and WHM rip through those mobs so fast it doesn't matter. Single target boss fights, this shouldn't be an issue though. And if you are missing positional a due to unneeded tank movement that's not an issue with your playing... Though it does increase frustration.