To your point about military specialties, the range of military occupation specialties that an enlistment candidate is qualified for is determined by the enlistee's ASVAB score at the time of enlistment, not their performance during training. It is true that during training, enlistees undergo further testing (DLAB, etc...) to identify candidates for specialized and rare positions like linguists and such. And some lucky, high-scoring enlistees are given their choice of jobs prior to enlisting as an incentive for signing on the dotted line. But the bulk of enlistees are already pre-destined for an assignment, based on the needs of the service -based on body-count, not test scores-, the day they step off the bus for basic training.
