Seller Chain pulls dimensions directly from Amazon. Amazon may often do a Cubiscan on an item and resize the item into another tier size. If the items size is the result of a Cubiscan editing the listing dimensions will not work to change the items tier size.
If editing the listing does not work the only solution is to contact seller central support and request a new scan. Amazon will require at least one unit in FBA to perform the scan. The scan take between one to three days.
If Amazon confirmed a change of dimensions it takes about 7 days for that change to reflect to shipment plans. Amazon Tier Size Chart