Difficult to say, but I’ve noticed when multiple combos of skulls appear at once, the damage separates each group of 3, not just adds to it.
It sounds like in your case it was 9-in-a-row, correct? My theory is this.
3 = First Troops Damage. (10)
4 = FTD +1 (11)
5 = FTD +2 (12)
6 = FTD +3 x2 (26)
In this theory, it both separates AND adds to the total damage. I couldn’t be sure of this without proper testing, but I believe it would explain the confusion. Basically, each unique set of 3 skulls counts as a separate combo, but in the case of the combo it counts the connecting skulls as well.
With this theory in mind, your new total should be…
10 Attack x 3 combos = 30 (+ 6 per combo) = 48.
That make any more sense for you? Once again, not 100% on the mechanics, but in theory it explains the damage you took. If someone more actually in charge decides to offer there reasoning, they could probably explain things far better on top of confirming the damage layout.