Damage Reduction

Damage Reduction
A creature with this special quality ignores damage from most weapons and natural attacks. Wounds heal immediately, or the weapon bounces off harmlessly (in either case, the opponent knows the attack was ineffective). The creature takes normal damage from energy attacks (even nonmagical ones), spells, spell-like abilities, and supernatural abilities. A certain kind of weapon can sometimes damage the creature normally, as noted below.

The entry indicates the amount of damage ignored (usually 5 to 15 points) and the type of weapon that negates the ability.

Some monsters are vulnerable to piercing, bludgeoning, or slashing damage.

Some monsters are vulnerable to certain materials, such as alchemical silver, adamantine, or cold iron. Attacks from weapons that are not made of the correct material have their damage reduced, even if the weapon has an enhancement bonus.

Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters. Such creatures’ natural weapons (but not their attacks with weapons) are treated as magic weapons for the purpose of overcoming damage reduction.

A few very powerful monsters are vulnerable only to epic weapons; that is, magic weapons with at least a +6 enhancement bonus. Such creatures’ natural weapons are also treated as epic weapons for the purpose of overcoming damage reduction.

Some monsters are vulnerable to chaotic-, evil-, good-, or lawful-aligned weapons. When a cleric casts align weapon, affected weapons might gain one or more of these properties, and certain magic weapons have these properties as well. A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that match the subtype(s) of the creature.

When a damage reduction entry has a dash (-) after the slash, no weapon negates the damage reduction.

A few creatures are harmed by more than one kind of weapon. A weapon of either type overcomes this damage reduction.

A few other creatures require combinations of different types of attacks to overcome their damage reduction. A weapon must be both types to overcome this damage reduction. A weapon that is only one type is still subject to damage reduction.

Ammunition fired from a projectile weapon with an enhancement bonus of +1 or higher is treated as a magic weapon for the purpose of overcoming damage reduction. Similarly, ammunition fired from a projectile weapon with an alignment gains the alignment of that projectile weapon (in addition to any alignment it may already have).

Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury type poison, a monk’s stunning, and injury type disease. Damage reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.

Attacks that deal no damage because of the target’s damage reduction do not disrupt spells.

If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.

Please note the red highlight makes no mention of creatures with alchemical DR actually causing that type of damage.
Whereas for every other source of DR, highlighted in orange, it states that they do cause damage as per their DR type.

So, for example, creatures with DR silver are not necessarily dealing silver damage - yes Ajax that is you.
This was a mistake during the battle with the Nightwalkers. Apologies, it will not happen again

So there did the idea that they did cut through the DR come from ?

I think it was just an assumption based on other DR types

Bugger