Back in the day when Tier lists were just becomming a thing the people putting them out used actual anecdotal evidence and real game testing and comparisons to make them for each class. Later several top performing players of each class and spec would contribute to form a sort of a meta aggregate Tier list for more accurate data because lets face it not EVERYONE is going to be a top performer for each spec.
Nowadays however what I have come to realize is that Tier lists are mostly inaccurate and just thrown together with sim data and theory crafting.
I've personally played some of the WORST class specs and done some of the MOST damage and actually saved some runs. Which led me to question what is even the point of these tier lists in the first place?
IF you roll a new character and just want a quick idea which spec is doing well right now you might think to yourself let me do a quick google search of a tier list so I'm not wasting my time and energy later with a class that is currently poorly tuned. Then you later find out the classes that scored worst are the ones topping damage meters later on.
The old axiom of "Play what you want" comes to mind. Because if a spec and class just "clicks" in your mind and you enjoy playing it then you will outperform most other players or classes regardless of where your spec falls on the tier list.
This is the trouble with Tier lists.