In these lines from "My Last Duchess," the Duke is speculating on why the Duchess blushed ("spot of joy") while the portrait was being painted. The Duke notes that the Duchess was too flirtatious ("Too easily impressed"). Whether the Duke's insecurities are justified is uncertain. But since the Duchess is not there to defend herself, we only have the Duke's interpretation. The Duke is judgmental and the way he objectifies his wife leaves the reader with the impression that he is, to use a pun, painting an unfair portrait of his late wife. When Pandolf says "Her mantle laps / Over my lady's wrist too much," he is essentially telling her to show a little more skin. The Duke notes that her blushing "'twas not / Her husband's presence only." So, the Duke suspects a flirtation between the Duchess and Pandolf. The Duke suggests his wife was always a flirt. The Duke believes that her blushing in response to Pandolf's request to show more skin reveals a provocative, seductive behavior rather than humble embarrassment.