How we cite our quotes:
"Altered beyond his knowledge." Anne fully submitted, in silent, deep mortification. Doubtless it was so, and she could take no revenge, for he was not altered, or not for the worse. [...] No: the years which had destroyed her youth and bloom had only given him a more glowing, manly, open look, in no respect lessening his personal advantages. She had seen the same Frederick Wentworth. (7.34)
Anne sees the change in her own position as an outward sign of her inner sadness; one could expand from that to say that Wentworth hasn't been as full of regrets as Anne, and that's why he looks so good. But it comes out later that he's been just as upset about the way things went as she was – so why doesn't that show in his looks the way it does for Anne?
"He [Mr. Elliot] looked at her [Anne] with a degree of earnest admiration, which she could not be insensible of. She was looking remarkably well; her very regular, very pretty features, having the bloom and freshness of youth restored by the fine wind which had been blowing on her complexion, and by the animation of eye which it had also produced. It was evident that the gentleman, (completely a gentleman in manner) admired her exceedingly. Captain Wentworth looked round at her instantly in a way which shewed his noticing of it. He gave her a momentary glance, a glance of brightness, which seemed to say, "That man is struck with you, and even I, at this moment, see something like Anne Elliot again." (12.6)
It takes Mr. Elliot's gaze for Wentworth to realize that Anne's getting her looks back – in this matter he's persuaded by another's opinion rather than by his own independent judgment.
He did justice to his very gentlemanlike appearance, his air of elegance and fashion, his good shaped face, his sensible eye; but, at the same time, "must lament his being very much under-hung, a defect which time seemed to have increased; nor could he pretend to say that ten years had not altered almost every feature for the worse. Mr Elliot appeared to think that he (Sir Walter) was looking exactly as he had done when they last parted;" but Sir Walter had "not been able to return the compliment entirely, which had embarrassed him. He did not mean to complain, however. Mr Elliot was better to look at than most men, and he had no objection to being seen with him anywhere." (15.12)
At last we see that Sir Walter has principles – he won't make the polite reply if he doesn't believe it. This also suggests that he believes wholly Mr. Elliot's remark that he hasn't changed, and doesn't in the least suspect that Mr. Elliot might not mean it.