Here is a clip of two successive attack rounds during a single combat. All parameters are the same; nothing is omitted. I didn't pick up any object, get my out shield or change stance between attacks. (The wildly differing results are clearly due to the differences in end roll, so that is not in question.) However, I note that the MM on the second, more successful attack actually dropped from 105 to 101; yet both attacks are identical in every way except for the end roll. I am at a loss to explain this, unless there is some random factor involved in MM determination. Any ideas?

>>stance offen
> punch giant left leg
You are now in an offensive stance.
H>
You leap from hiding to strike!
You make a precise attempt to punch a stone giant!
You have excellent positioning against a stone giant.
UAF: 344 vs UDF: 317 = 1.085 * MM: 105 + d100: 2 = 115
... and hit for 2 points of damage!
Awkward punch gently brushes the left thigh. It's like a first date!
Roundtime: 4 sec.
R>hide
Roundtime: 3 sec.
You attempt to blend with the surroundings, and feel confident that no one has noticed your doing so.
HR>stance offen
> punch giant left leg
You are now in an offensive stance.
H>
You leap from hiding to strike!
You make a precise attempt to punch a stone giant!
You have excellent positioning against a stone giant.
UAF: 344 vs UDF: 317 = 1.085 * MM: 101 + d100: 88 = 197
... and hit for 50 points of damage!
Furious fast fist forces fibula fracture! Foot flies free, falling forward five feet forth!
The stone giant rumbles in agony as she teeters for a moment, then tumbles to the ground with a thundering crash!
The stone giant is stunned!
Roundtime: 4 sec.
R>
A stone giant throws her head back and roars, shaking off the stun.<<

"For the female of the species
is more deadly than the male."

There is a random factor in MM. The better the EBP of the target is, the larger the random factor removed from MM can be. One of the reasons hiding works so much better than open UAC is the way it cuts down the size of this factor.

Stance is an important part of EBP and you can see drops of 40 rather than 4 from this factor if you take on turtled critters from the open but its the consequence of critter stance on EBP that affects MM rather than the direct effect your stance has on it. If you uphunt to the extent that you cease getting pushdown, you'll see massive MM reductions because the critters get much better EBP as they outlevel you too.