One way to view nutrients is "flow" instead of "level", just like a river. By keeping a very high "flow" of nutrients by providing heavy inputs and exports, it's like a fast flowing but shallow river. This type of flow is how natural water systems work (reef, lake, etc). They might measure "low" nutrients, but their internal flow of nutrients is high; so high it's hard to believe.

A fast flow is capable of filling up voids quickly when needed, like if you put a bucket down into the flowing river. The void can be corals growth, of course. So by inputting more (feeding, dosing, etc) and exporting more (macro growth, etc), the flow is large and available for coral growth when demanded.