This (and the Interest) description is not quite right. If a user has an interest in state bills about agriculture, they will only receive notices of new bills around agriculture. If they subscribe to specific state bills (an "item"-type Interest) then that Interest will have multiple subscriptions for actions and votes on that bill only.

In other words, an Interest is either about 1 or more searches on lists of items, or 1 or more feeds of updates centered around a specific item.