Nostalrius Begins - Quality wow vanilla realm (1.12)

Hi,
Siphon Life in conjunction with Shadow Mastery does more damage to beasts than intended, and less damage than intended to humanoids.
Proof: Rank 1 siphon life, rank 3 shadow mastery, no spell power. http://postimg.org/image/rjypoqi0d/

Theoretically my siphon life should hit for 150 x 1.06 = 159.

Vs Humanoid: Tick damage is 15 http://postimg.org/image/fxnsf39gt/
, adds to a total of 150 http://postimg.org/image/rj5cwgg63/ ;

Vs Beast: Tick damage is 18 http://postimg.org/image/w4d7qrgu1/
, adds to a total of 180 http://postimg.org/image/cvsiwxf57/ ;

...And no, it is not because the mob is or is not shadow-resistant. If something is x-resistant, it has a chance to resist a part of the damage and it would be shown in the combat-log.

I did notice a slight difference, 59 vs 58 ticks when I tested this in durotar this morning, but after 10 or so minutes everything became 58 ticks. No idea what's going on with it.