Jabba's lead has more damage reduction for hutt cartel which adds together with the datacron, causing the 1 damage you've seen.
Don't know about the subsequent hits damage reduction though. Edit: just thought, does the assist call have a damage reduction built in? Like "all allies assist doing 30% less damage". That could add together with the datacron, just like jabba's lead.
If you watch the times when the 75% damage reduction is not happening, the hits are in the thousands. If a hit would be 4000 without the reduction, it should be 1000 after the reduction. For a 75% reduction to result in a hit that does one damage, the hit would have to do 4 damage without the reduction. That is simple math. If you are saying they are taking a 30% reduction and adding it to a 75% reduction to get a 105% reduction, that is an incorrect way of calculating this function. Each percentage should be calculated in series. First do the 30% reduction = 70% damage. Then do a 75% reduction of the 70% = 17.5% damage.
