ASVAB scoring is based on an Item Response Theory (IRT) model. IRT is a theory that enables test questions and examinee abilities to be placed on the same scale, thereby allowing tests to be tailored to the specific ability level of each examinee and scores to be expressed on the same scale regardless of the combination of items that are taken.
The IRT model underlying ASVAB scoring is the three-parameter logistic (3PL) model. The 3PL model represents the probability that an examinee at a given level of ability will respond correctly to an individual item with given characteristics. Specifically, the item characteristics represented in the 3PL model are difficulty, discrimination (i.e., how well the item discriminates among examinees of differing levels of ability), and guessing (i.e., the likelihood that a very low ability examinee would respond correctly simply by guessing).