How can this be explained mathematically?

From http://www.southperry.net/showthread.php?t=31437
Simplified to remove the if stuff, percentage based damage only.

Code:

int v5;
  double PDDMultsqrt;
  double v11;
  PDDMultsqrt = sqrt(PDDMult); // 70.71
  v5 = (a5 * PDDMultsqrt) / 100 + PDDMultsqrt; // a5 is some other defensive buff, I don't know what.  Anyway, if it's 0, this will be v5=70
(if moblevel > charlevel there's additional work here to reduce the value in v5)
  v11 = (100 - v5) * mob_base_damage * 0.01; // 30 * mob damage * 0.01
  return v11;

v5 is effectively truncating the percentage to a whole number (in this case 70%.