Blast's hidden 20% crit rate

Actually, as long as we're discussing Blast and its criticals, does anyone here know what it is that causes Blast's crits to come out as the exact same number every single time against boss monsters. Also, most monsters outside of Victoria Island seem to get hit for the same numbers on the first 2 critical hits of Blast but not the third one. No matter how I try to explain it by running through the damage formulas, I can never figure it out.

so far, i believe the only explanation is "magic" something is messed up with the RNG and blast, probably like something is messed up with 4-hit techniques like soul driver, or accuracy problems where the damage is the same.