No mistake.
Trolls (both types) take Large Creature crits instead no normal crits and they get suvival instinct., I did not want to give either of those abilities to a Half-troll PC. Therefore, I thought it through and decided that Reverberating Strength was good trade-off for a half (or quarter) troll as opposed to receiving the more powerful abilities.
This was a conscious and specific decision. And remember, while reverberating strength is only freely available to those with bit a bit of troll blood, there is nothing to say that you give that ability to trolls as well (though I would make them pay for it in DP).
