They may have Married over the Border in Lincs and Mary may have been from Lincs;
Marriage IGI Extracted;
John KEYWORTH
Mary POWELL
27 July 1783, Haxey, Lincolnshire
Trish

EDIT;
IGI has a Mary POWEL christened 08 May 1760, Epworth, Lincoln, England
Parents JOHN/MARY
Siblings on the same batch born 1746/1767
Submitted Marriage;
John POWELL
Mary HULL
25 February 1745, Epworth, Lincolnshire
Christening date of 1760 matches age at Death for Mary KEYWORTH Burial 13 May 1828, age 67 in Walkeringham, Notts. She would be born c 1761.