The issue is that a good portion of what makes a difference between one percentile and another isn't the player it's the party... You can take a take for example the 100th percentile player and put him in a group of randoms and his performance will be no where even close to that 100th percentile.
There's so many things that fflogs doesn't tell you that can have a significant impact on your performance. And a lot of them can be as simple as which direction is the boss facing or where is the boss being tanked?. A badly positioned boss might mean you have 10% less uptime. A boss thats constantly being turned might cost you a significant amount of positional bonuses.
A party member that's stood right next to you might then force you to move and lose cast time where if he was in a better position you wouldn't have to move... lost gcds..
Badly stacked ground aoes can make the difference between taking 2 steps to get out of or taking 7 or 8.. lost gcds.
That player with the stack marker forcing everyone to move away from the boss to stack on him instead of him running to the group on the boss.. more lost gcds
There are quite literally an almost endless number of things that can really impact performance none of which are ever represented in fflogs...
Then you have things like other party members and how well they use there buffs. Is chain start being applies with foe and trick attack muliplying the damage boosts quite heavily or are they all being used sporadically at random intervals.
All of these things might slash your performance in half and there's absolutely nothing you can do to control it.
Player percentiles are essentially worthless. Because it's generally the entire group that makes that number what it is. You could take a 10th percentile player put hi in a 100th percentile group and guarantee he'll do better even if he plays exactly the same....
And like I said at the start. Take a 100th player put him in a 10th group and his performance will be nowhere close..