Manufacturer coverage claims are measured on a smooth, uniform drawdown card under lab lighting, applied at a specified wet-film thickness. Real walls are none of those things: they carry old patches, drywall mud with different porosity, and years of sun exposure that changes how evenly a single coat lays down.
The most common failure we see with a single coat is flashing: patches of drywall repair or old touch-up paint that absorb the new coat differently and show through as a slightly different sheen once it dries. It is rarely visible wet, which is exactly why it surprises people who tried to save a coat themselves.
Color change amplifies the problem. Going from a mid-tone to a light color, or the reverse, on a single coat almost always leaves visible roller and edge lines even when the total mil thickness technically matches the can's claim, because coverage and hide are not quite the same property.
Two coats also just last longer. A second coat evens out film thickness across the whole wall, and a more uniform film resists scuffing and washing better over the years ahead. We treat the one-coat claim as true under lab conditions and irrelevant to the job in front of us.
