Dante is caught between two questions pressing with equal force — like a man between two hungers equidistant from two dishes, who starves rather than choose. The first: was Piccarda's place here unjust, since her will never broke? The second: did Plato tell the truth when he claimed souls return after death to the stars from which they came?
Beatrice addresses the Platonic question first: the souls do not actually dwell in their respective spheres permanently. They appear here — in the Moon, Mercury, Venus, and so on — as a concession to Dante's finite understanding. His mind needs the visual, spatial form; the truth is that all the blessed dwell together in the Empyrean, but they manifest in the sphere whose quality most closely corresponds to their particular beatitude. Plato's myth of souls returning to stars was therefore not entirely wrong — it pointed at something real, even if his literal astronomy was false.
On the question of Piccarda's will: Beatrice distinguishes between absolute will and conditioned will. Piccarda's absolute will — the deepest orientation of her soul — never consented to abandoning her vow. But her conditioned will, the will that operates in the actual circumstances of fear and physical force, did yield. She could have fled, or fought, or died resisting. She did not, and that matters. The distinction is subtle and honest: she is not blamed, but Dante must understand that purity of will is not simply what one wishes in theory; it is also what one does under pressure. The souls in this sphere are not condemned but placed lower precisely because their constancy, however innocent, was not absolute.