With the last update to Final Fantasy XIV, the team made some changes to the Pandaemonium Abyssos: The Eighth Circle (Savage) raid, changes that have left some in the community questioning exactly why this was made. According to Yoshi-P in a post on the FFXIV, basically, maths and player skill forced the change.
Naoki Yoshida, the director and producer on FFXIV, explains the reason for adjusting the HP on fights in the Savage raid. Raising the HP in the raid, though, had some undesired effects according to Yoshi-P, such as some jobs simply being excluded during player recruitment into the raid, as well players getting disgruntled over the harsh cliff in difficulty.
As far as how the miscalculation for the balancing act here happened, it starts from one very important premise: players are just better at Final Fantasy XIV than the developers. This is actually a common refrain I hear from studios all the time, so it's not uncommon to hear from one as large as Square Enix. Players simply have more time to min-max in real-world situations whereas developers are busy...developing.
Additionally, they typically add more to the boss HP values in testing, though even then the rebalancing team is operating on the premise that players themselves are just better at playing the game.
"The team responsible for balancing boss fights does so without debug commands and at the appropriate item level, employing available materia, foods, and medicines while experimenting with mitigating actions and various job compositions. Yet we recognize that player skill far exceeds our own. If we were to ship content with the same values which challenged our battle team, the top raiders would be deprived of that by-the-skin-of-your-teeth victory in the initial week of release."
Yoshi-P gets into the maths at the center of it all, too, stating that balance test clear values are usually +1-2% HP on bosses, with the recent raid being more challenging than usual thanks to its proximity to the main story quest and the character surrounding it. The end result should, in theory, be a "satisfying, hard-won victory" for the players.
Why the team chooses to balance the Duty itself instead of Jobs, it comes down to the variance in job mechanics, numbers, and more in each battle. However, because of the unique issue with the Abyssos: The Eighth Circle (Savage) raid, it favored more damage-dealing classes over others, which left players out in the lurch who were in lower DPS jobs.
As such, because the cause of the issue was inherently with the duty itself and not with jobs perse, it made more sense to balance the duty to deploy the fix.
It's a pretty interesting post and gives some insight into how these massive tentpole encounters are created and balanced before being released. You can read the full details on the Final Fantasy XIV website.