Hope has the cleanest emblem of the three theological virtues and one of the oldest in Christian use. The anchor comes from Hebrews 6:19, where hope is called an anchor of the soul, sure and steadfast, and it appears scratched on tomb slabs in the Roman catacombs from the second and third centuries, well before the cross became the ordinary Christian sign. It survived into the later repertoire unchanged, and it is still the one attribute a viewer can read without a handbook.
The image works because it inverts what hope is usually taken to mean. An anchor does not lift; it holds, and it holds by being fixed in something the ship cannot see. That is exactly the definition the tradition gives, and which the atlas records elsewhere: a movement of the will toward a future good that is difficult but possible, resting on an assistance outside itself. Optimism would be drawn as wings. Hope is drawn as ballast.
Painters did also give her wings, and a crown reached for above, and hands raised toward something outside the frame. Giotto's hope at Padua is winged and rising, which is the other reading and the one that stresses the movement rather than the ground of it. Ripa keeps the anchor.
The atlas draws the anchor. It is the older attribute, it belongs to hope alone, and it reads at small size, which the raised hands and the distant crown do not.