Answer from Danavir Goswami
There are many factors that could influence the moon’s light. Distance, heights and angles of both the sun and moon in relationship to earth along with the vantage point of the observer. Besides that there is another feature going on; i.e. the moon’s nectar is drunk by the devas and pitris which affects its glow. Here is a brief excerpt from the Chapter 141 of Matsya Purana:
The Sun acts daily (in the bright fortnight) as a feeder through His Susumna ray when the store of lunar ambrosia is all drunk out by the Devas and the Pitris. The Moon waxes in his phases day by day by thus being fed through Susumna ray in the bright fortnight. The Moon wanes in the dark fortnight and waxes in the bright fortnight. The moon is nourished thus by the Sun. The Moon looks full and white on the full moon night (Purnamasi). In this way, the Sun by means of his single ray increases the Moon and makes it full of nectar. The Devas first drink the nectar of the Moon; then the Sun drinks. The Sun drinks every day one digit and does so for fifteen days; He, again in the bright fortnight, fills it up by His Susumna ray.—21-25.
The phases of the Moon that wax in course of the bright fortnight fed by Susumna, wane during the dark fortnight. In this way the Moon continues to wax and wane, consequently, the full-moon is called the receptacle of nectar. He is luminous with the fifteen nectar-giving phases. He is, therefore, called Pitriman.—26-28.