Percentage of water extracted from the rocks, would be my guess. With death valley, these rocks are subjected to consistant high temperatures, and it is this high temperature ongoing thrughout the majority of anual climate measurment within this valleys milimeunim, that all but infractional moisture content, these rocks have, (or, do not have!), then bakes out. The climate bakes these surface rocks, down to the core, and it is from this slow baking oven type of process, that ballast rocks can finaly then be calculated by thier actuall various weights. This final weight amount is also helped in the dry rock, by the air preasure, (being far below the sea level), which helps by squeezing the remaining moisture from each rock, like that of a sponge, squeezrd out, for any future absordations.