It is very unlikely for both Stealth Rock not being up and Amoonguss not using Spore. That situation is heavily geared towards Carracosta for the sake of argument and will often not happen in an actual game. The point is that Life Orb does better against Amoonguss. Not that Liechi Berry can get past Amoonguss if certain field conditions are not present in any given match.
As for the damage calculation tidbit:
It might be the case of this damage calculator being retarded.
<&DTC> is kalashinov retarded