I have some anomalies of my own:
Skull (14) (stunned) vs Sparkling (5) = 19 dmg 14 + 10 - 5 = 19
Skull (15) vs Sparkling (5) = 20 dmg 15 + 10 - 5 = 20
Skull (15) vs Sparkling (stunned) (4) = 21 dmg 15 + 10 - 4 = 21
Skull (15 +3 with scourge) vs Sparkling (frightened) (5) = 23 dmg 15 + 3 + 10 - 5 = 23
Skull vs Sparkling seems to always use base 10.
Same with Magiciain of Alahan:
Skull (15) vs Magician of Alahan (6) = 19 dmg
Everything else that I have noticed uses base 9.
I haven't done many tests with hard boiled yet but I'll take your word for it