Some Full Moons are not 100% illuminated, as seen from Earth. If that is the case, our Moon phase page will display a slightly lower value at the moment of the Full Moon, such as 99.9%.
The Sun always illuminates one half of the lunar surface, except during lunar eclipses. The phases of the Moon are determined by our viewing angle: at New Moon, we look at the dark side of the Moon; at Full Moon, it's the illuminated half. Read a detailed explanation of Moon phases
The reason why we cannot see the entire illuminated hemisphere of the Moon during some Full Moons is that the Moon's orbit around Earth is inclined at an angle of about 5° in relation to the Earth's orbital plane, the imaginary disk whose outer edge is defined by our planet's orbit around the Sun and which astronomers call the ecliptic. So, at some points during the lunar month, the Moon is “above” the ecliptic, and at other points, it is ”below” it. For an illustration, please see the section about lunar nodes in our explanation of lunar eclipses.
If the Full Moon occurs at a moment when the Moon is particularly far above or below the ecliptic, we do not exactly face the illuminated part of the Moon but view it at a very slight angle—an angle that is large enough to diminish the illumination percentage by a tiny amount.
That said, while our algorithms account for it, this effect is not actually noticeable when looking up at the Moon. In fact, the Moon generally looks full a few days before and after the Full Moon phase, when the illumination is considerably lower than 100%.