Weapon damage types

Hard to find an explanation as to why weapons do different damage types.

Example: one BLP will do both Pen and Burn dmg while another BLP type will do Impact and Electric. Another BLP weapon, DOA Slugstorm, per Entropedia, shows no specific dmg type done, Why the differences in same weapon type? And, are certain weapons better for a specific mob based on the damage types they do?

I appreciate any info you can share, or point in the direction of an explanation on forum I can review myself.

Thanks to all response's

The damage type doesn't matter except for PVP fights. Then you must align your damage types against the protection you think your opponents will have.