Prior to the four of you running the three Elder Rifts where you all put in 10 crests to each of them (which should, indeed, result in 120 bonus embers) had any of you already run Elders and used some crests?
For example, the player that received 115 embers from your 10 crest runs… if they had previously run Elder Rifts where they had put in 5 crests (e.g. one run putting in 3, one run putting in 2) they would have earned 5 bonus embers. When they subsequently ran the group Elders, that means they would only have 115 bonus embers left to earn from the 120 weekly cap.
So…
Had any of the four players already run Elders with crests prior to your group runs?