Turn 4 DPS numbers are highly variable depending on how long you spend in each phase, RNG crits, fight approach, and other things. Our BLM has varied up to 50 DPS on a Turn 4 parse while he basically did the same thing both times (I think he misfired like 1 Flare on the low parse though, but 1 Flare does not make a 50 DPS difference). In addition, the way the damage output is done in Turn 4 leads to a situation where the performance of other classes will directly impact affect how long the BLM has on each phase.
As a simple example, if a BLM has a lower parse, it could be because the other classes did more AOE damage during phase 1/4, and have literally nothing to do with the BLM's performance themselves. Like all of a sudden the Dragoon remembered how to play their class and hit a BFB'd DFD during P1/4. With two button presses of another class, the BLM could lose 15-20 DPS. Or hey, the Bard remembered to use BV on their FR (or remembered to use FR at all instead of mana/TP?). Boom, 5-10% total DPS gain.
Personally, I virtually ignore T4 parses for any reliable data unless you're consistently getting the same result over like 10-20 attempts after your group has stabilized the overall fight pattern. Even then, I would ignore DPS numbers by "same class in different groups" unless they have the same comp and use the same fight pattern (kill order, etc) ... which doesn't happen.
For example, some groups could have the BLM kite the soldier in P5 or P6. This is a DPS loss for the BLM. A group could have the mages in P6 attack the Dread until the Knights are dead, then AOE the Soldiers + Spiders. This would inflate BLM DPS. Shrug.
Edit: T2 is a bit of a mess since different targets will have different damage resistances from the mechanics. T1 is the most stable ST DPS check, unless you're cool enough to deep dive into parses and check DPS against individual mobs like each miniADS separately, etc.