There are 102 Unholy Deathknights in that stat you've linked...compared to 10 times the amount for Frost DK's and over 15 times the amount for Assass Rogues.
In other words to compare 75th percentiles with such a small sample size for Unholy Death Knights is pretty meaningless.
Let's give another crude example to reiterate the same point about why that particular statistic is fairly pointless to gauge how well DK's are doing.
If you look at the 99th percentile it shows that Survival hunters as a viable top DPS spec...but there are only 30 Survival hunters parsed on M Krosus, of which the top spot hunter counts for the top 4 percentiles! Unless there is 1000 logs of different hunters doing similar DPS, you could argue for that particular parse, that hunter got extremely lucky with procs. Of course we'll never know since there are not enough of them out there that would play such a damned spec.
Anyway here are my logs showing me at the bottom of the DPS meters on M Skorpyron...as requested
https://www.warcraftlogs.com/reports...pe=damage-done.
And here are my logs on Heroic
https://www.warcraftlogs.com/reports...pe=damage-done.
Lastly my point about Mythic Krosus stands. Due to the natures of Orbs, add killing, soaking etc you need some classes to average pretty highly to carry the ones that get bad RNG and can't push above 600k DPS. To simply state that all that's needed is around 620K dps is a fallacy, when in fact you need a percentage of players OVER the 700K to account for bad RNG for your ranged dps, who tend to really suffer on that fight.