In comments under an otherwise persuasive answer, KRyan mentions that armor proficiencies are always by category, i.e. armor proficiencies only come as light, medium, and heavy armor proficiency.
This is in contrast to weapon proficiencies, which come as simple, martial, and exotic, but also come as monk weapon proficiencies, elf weapon proficiencies, and others, as well as any number of individual weapon proficiencies.
I can't think of any counterexamples or of anything explicit: Is there good evidence as to whether armor proficiencies are always by category?
Races of Stone does introduce another subcategory of armor, “exotic.” Exotic armors are still light, medium, or heavy, and you need both the proficiency for the weight category, and Exotic Armor Proficiency. If I recall correctly, Exotic Armor Proficiency does apply to a specific armor and not a category, but since it’s orthogonal to the weight question (and you still need the proficiency in the weight category too), it seems like not what you’re asking about...
Exotic armor appears to me to be introduced as a special case. I am open to answers which reason from there to say that you could have (say) proficiency in only studded leather armor, but I think there is a non-trivial explanation that has to come with that.