Not a paradox
The FEC is for things we can see, and those happen to be the things horses either do, or don’t develop good/better/best immunity to - ascarids and strongyles. Horses get classified as generally low, moderate, or high shedders, and low shedders can have a high FEC at some point in their life due to anomalies we don’t always know.
They don’t show you pinworms, bots, or tapeworms, and those are things horses don’t develop immunity against. BUT, you don’t have to deworm often for those more than twice a year (there are always exceptions) due to their lifecycle.
If you have a historically low shedder, then no, you don’t have to do FECs twice a year, or even once. Deworm Spring and Fall and you’re good.
But a horse’s status CAN change, especially as they enter Sr years, and you should have something in place to monitor their status at least every few years.
If your horse is historically a moderate or high shedder, then you need to deworm more than twice a year. The timing of that depends on your local climate. For the SE, that additional 1-2 dewormings is likely to be in the Winter. For the NE and Midwest, that’s likely to be Summer.