Sandy soil has very little to hold nutrient with. Water moves through it in minutes, and dissolved fertiliser goes with it.
So a single heavy spring application does what it looks like it should for about a fortnight, and then the bulk of it is below the root zone and gone. The lawn greens up, fades, and you conclude the product was no good.
The same total quantity, split across four to six lighter applications, behaves completely differently. Each one stays within reach long enough for the grass to actually use it. Same spend, entirely different result.
That is the whole case for a programme rather than a one-off, and it is why we price it by the year.