A Question on Critical Threat Ranges

[Prux]

A keen falchion has a critical threat range of 15-20.

If a character wielding a keen falchion was to take improved critical (falchion), shouldn't the threat range then be 12-20?

I'm using herolab and when I add the improved critical feat the threat range doesn't alter.

Any ideas?

[Jeraa]

Improved Critical and Keen do not stack.

Quote:

Special: You can gain Improved Critical multiple times. The effects do not stack. Each time you take the feat, it applies to a new type of weapon.

This effect doesn't stack with any other effect that expands the threat range of a weapon.

Quote:

Keen: This ability doubles the threat range of a weapon. Only piercing or slashing melee weapons can be keen. If you roll this property randomly for an inappropriate weapon, reroll. This benefit doesn't stack with any other effect that expands the threat range of a weapon (such as the keen edge spell or the Improved Critical feat).

[Prux]

Jeraa wrote:

Improved Critical and Keen do not stack.

Quote:

Special: You can gain Improved Critical multiple times. The effects do not stack. Each time you take the feat, it applies to a new type of weapon.

This effect doesn't stack with any other effect that expands the threat range of a weapon.

Quote:

Keen: This ability doubles the threat range of a weapon. Only piercing or slashing melee weapons can be keen. If you roll this property randomly for an inappropriate weapon, reroll. This benefit doesn't stack with any other effect that expands the threat range of a weapon (such as the keen edge spell or the Improved Critical feat).

Thank you.