I investigated into the bonus vs animal bug and found something much more scary. Here is my results
Noble Difficulty
+10% vs barb
Scout vs. Barb lion
2.10 vs. 0.95
scout 1 strength
+110%
=2.10
Lion 2 strength
/2.1 (110%)
=0.95
Scout vs. Civ lion
2.00 vs. 1.00
Again if both increases the strength of the scout by 100% but halves the lion.
Prophet vs. Barb lion
2.20 vs. 1.81
Adds the 10% to the prophet due to bonus vs barbs and reduces the lions strength (2.0/1.1(10%)=1.81)
Prophet vs. Civ lion
2.00 vs. 2.00
As it should be
Monarch difficulty
No bonus vs barbs
Scout vs. Barb lion
2.00 vs. 1.00
Scout vs. Civ lion
2.00 vs. 1.00
Prophet vs. Barb lion
2.00 vs. 2.00
Prophet vs. Civ lion
2.00 vs. 2.00
Now the scout gets bonus a bonus and the lion get a penalty, then something hit me..
Warrior (with elf slaying) vs. Elven warrior
2.80 vs. 1.42
the warrior gets the +40% vs elves as usual BUT the elven warrior also gets a 40% penalty (2.00/1.4=1.42)
Conclusion
ALL bonus versus other units act as both a bonus to the unit and and penalty to the other unit.
This really needs to be fixed
in civ-promotions like anti melee, city raider, and slayer promotions from FFH
are calculated as reducing your opponents strength
most likely, you had 1-2 strength promotions or some near equivalent with +20% combat
(combat one or divine hammer, ect for prophet)
(combat two for the warrior)