A famous masterpiece by Raphael – why did he call his painting the Sistine Madonna?

The figure on the left has six fingers on his hand. It is the hand of Saint Sixtus. The number six in Latin is "sixtus." It is in these six fingers that the cipher of this Raphael painting's title is found. Pope Sixtus II, who was martyred in 258 A.D. and became anointed as saint, asks the Virgin Mary for the protection of all those who pray to her before the altar. On the right, Saint Barbara's posture, her face and downcast gaze express serenity and reverence. Deep in the background of the painting, there are figures of angels, barely visible in the golden haze, thus enhancing the overall elevated atmosphere.