Its kind of interesting, the Damage Table appears to be where this might be adding in as far as I can tell. Here's where it is in EqEmu:
https://github.com/EQEmu/Server/blob...tack.cpp#L4690
If I am reading this correctly, its taking your offense and subtracting some number from the table, then dividing that by 2, then doing a random between 0 and that number to add as an extra % to your damage.
So here's an example level 60 damage table entry:
{ 285, 23, 65 }, // 60
Those numbers are Max Extra, Chance, Minusfactor.
There's a 23% chance it just doesn't do anything on a hit. Then subtracts 65 from offense, which has GetAtk as a component, divides by 2, does a random between 0 and (offense - 65)/2 and adds to 100, compares to 285 to see which is smaller, then uses that.
I think based on that, this would lead to larger max hits if (offense - 65)/2 +100 is smaller than 285, because then you can't cap.
offense appears to be skill+atk+(2 * str-150)/3. At 225 Str, on an sk with a 225 skill cap, that would be (225 + (225*2 -150)/3 - 65)/2 + 100, which is 230, so under 285. Up to 110 additional atk would increase max damage. For a Rogue with Epic and 255 Str, it would be (250+(255*2-150)/3+40-65)/2 + 100, which is at 272.5, so up to 25 ATK would increase max damage. Rogues getting Raid buffed will usually have VoG and some combination of Aura of Battle/Grim Aura/Strength of Nature/Call of the Predator, which would make something like Avatar or Bard +ATK songs not add any additional max damage.
Additional +ATK would still impact the roll20 function, which was applied before the damage table as a multiplier between 0.1 and 2.0 that was dependent on the relative offense vs. ac rolls.
I bet this effect is why there are a lot of claims +ATK doesn't add to max damage but only skews the distribution, but you're seeing a +max damage boost from it.
What is really interesting to me is that the actual Offense skill doesn't seem to be used as part of the damage calc directly, just as part of the compute_tohit here:
https://github.com/EQEmu/Server/blob...ttack.cpp#L149
Whew, all this stuff is complicated. I wonder how much P99 changed it as well?