Its not exactly how much damage you do.
Its your build. This is my theory..
Guilds give a postive addition to assa/scout/steal plunder. This is because you get spy stats. SoS and Vol will increase your steal plundur because the stats increase by a ton. However, only with vols will the assa and scout plunder increase, that because the total plunder (in ally/self) is greater than guild, for SoS is not.
Now, this is thr confusing part... Troop buildings decrease the amount of plunder an assassination/scout/steal will earn. This is determined by how many/how great the stats are by the troop building(s).
This is just what I see in this mechanic.