Metrics usage seems to be fair & below are possible reasons for your observation.
D -> Link Name -Submit , Metrics -> Instances >> D -> Pages Report(thank you page), Metrics -> Page Views ; The difference is more than 20%
Probable reason - Submit instances may be higher in case you are tracking the link for client side form errors (mandatory field not filled, incorrect field format entry) & server side errors (page not found on Submit click)
D -> Link Name -Submit , Metrics -> Visits << D -> Pages Report(thank you page), Metrics -> Visits ; The difference is more than 20%
Probable Reason - Visits may be higher for thank you page in case visitor is accessing this page directly (in a separate visit) via a book-mark URL or using browser back button to reach thank you page.
You may want to capture a serialize event on thank you page for accurate measurement of form submissions...