It's just a two-tier probability process,
so every time the plugin needs to create an item it chooses a category, then chooses an item from that category.
If you had 5 categories and their probabilities were 1, 1, 1, 1, 6
the've all got 1 in 10 chance of being picked, except the last one which has 6 in 10 chance.
Same logic applies to items within.
If you have some category with probability 1 or more, and some item within it with probability 1 or more then yes,
your npcs should always get that item.
If they're coming up empty I'd guess they're being looted, or some other plugin is maybe clearing their inventory.
Since you're having problems, I'd stick to the default categories and items for now.
Maybe try custom stuff, if you want, once you know it's working as it should.