SA dice represent a rogue's ability to exploit anatomical knowledge to deal critical damage.
All classes have the ability to deal critical damage, a Rogue is just well prepared to exploit anatomical knowledge with martial prowess.
The Rogue can shiv his dagger through an eye slit on a full helmet. The flask of Alchemist Fire shatters against the eye slit and splashes fire all over the helmet, but chances are that none of it (or a very small amount) would get through the slit.
The rogue can maneuver his blade beneath the shoulder plate to pierce into the fighter's flesh. The Alchemist's Fire splashes against the breastplate.
Without looking at the die rolls necessary there, the Lion Totem Whirling Frenzy Barbarian with Power Attack and Headlong Rush is an optimised character path that requires step-by-step, thought out processes.
This is a rogue that picked up a flask.
Looking at die rolls.. assuming a d12 weapon, power attack at 8 and a 20 str.
d12+16+5's average damage is 27 x 2 for Headlong Rush is 54.
Not quite 120.
Sneak Attack damage is, first and foremost, simply mechanical extra damage. How you explain it is up to you. But as long as your explanation doesn't include a way in which Alchemist's Fire or Acid can inflict that extra damage, it's simply not a good explanation. Coz those things do extra damage in the hands of a Rogue, like it or not. It's up to you to change your view of how things work, not up to the rules to prove your view of things right.
That Barbarian has Lion Totem for a reason: so he can take a full attack on a charge. Include Whirling Frenzy, and maybe a Haste spell, in the equation, and you're looking at much more than 120 points of damage (if all attacks hit). Also, Str bonuses on two-handed weapons are x1.5.
BTW, the Barbarian example wasn't just taken from thin air, and doesn't really represent great optimization effort.
I have a Half-Orc Barbarian 8 just like that running around in my group. He has Str 22 (16 base, +2 racial, +2 levels, +2 Gauntlets of Ogre Power - not unusual), 26 in a Whirling Frenzy. He uses a simple Guisarme +1, and often receives a Haste
and an Enlarge Person spell prior to combat. Two feats (Power Attack and Headlong Rush) ensure he can, as a rule, one-shot whatever he can charge.
Damage potential on a full attack in a charge, Frenzying, Hasted, and Enlarged, with full Power Attack:
2d6 + 30, times two (Headlong Rush), times four attacks (two from BAB, one from Frenzy, one from Haste),
equals 16d6 + 240, or 296/round, not counting crits.
This is when all attacks hit (which they usually won't), and the player never Power Attacks for full, but still.