At first sight, I'd say that you forgot that extra gems give extra levels to the caster, and this is also valid for spell effects, like additional summoned creatures or penetration bonus. So a level 12 mage gets 4 extra levels when he is able to cast Master Enslave (lvl 8) with 12 pearls (4 extra, that's the maximum).
And local magic dominion could have an effect too.

But I've made some more tests and I must admit that something is wrong.
I sent two lvl 9 astral mages with 20 pearls each (but no items) in a province with 100 Vinemen (mindless, MR 5), 100 Spirits (undead but not mindless, MR 11) and 100 Hama Dryads (MR 15). None of them are "astral" units, so they have no special bonus against Master Enslave. Local dominion is absolutely neutral.
The first mage casts Master Enslave and the 2nd mage casts Vortex of Returning (so I can easily see the "losses").

First test : 37 spirits and 12 Dryads are lost. Second test : 24 spirits and 15 Dryads are lost. Third test : 32 spirits and 11 Dryads are lost. Vinemen are unaffected.
Hey, that's much more than expected indeed :
penetration 5 (base 4 for easy resistance + 1 extra for 2 lvl above 8) vs MR 11 = 7,1% chance,
penetration 5 vs MR 15 = 1,7% chance.

And I've another problem : a lvl 9 mage is not supposed to cast Master Enslave ! He spent 9 pearls but that's only 1 extra, so he had 10 levels, e.g. two above the minimum (8). His fatigue should be 800 divided by 3. In my tests he gets only 200 fat (or 800 divided by 4). So for some reason Master Enslave is treated as a level 7 spell !

Any thought ?