Easiest way to explain it:
In a certain job, you can define a "Maximum Health Value". Therefore, medkits can check the Max Health of a Job, and only allow you to heal up to that amout.
However, a "Maximum Armor Value" does not exist, therefore if I set the max armor for the armory to 200, every single job is able to buy 200 armor, including jobs that don't spawn with 200.