Found out why, you can mod it. Here's the snippet from creature_standard.txt in the object raws.
Attack definitions are formatted as follows:
[ATTACK:PUNCH:BODYPART:BY_TYPE:GRASP]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:punch:punches]
This means that the attack will use as much of the available tissue as possible, rather than, say, thrusting with a spike.
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
This causes all of the nails on the finger's of a given grasp to be used.
[ATTACK:SCRATCH:CHILD_TISSUE_LAYER_GROUP:BY_TYPE:GRASP:BY_CATEGORY:FINGER:NAIL]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:scratch:scratches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:SECOND]
This causes all of the teeth on a given head to be used.
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_CATEGORY:HEAD:BY_CATEGORY:TOOTH]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:SECOND]
[ATTACK_FLAG_CANLATCH]
Looks like you want to change ATTACK_PRIORITY on them.