I've been looking at the King's born in Tetbury and it's interesting.
There are a number of King's born about 1800 listed on the censuses - Charles, Elizabeth, Ann, Mary and James - but there are only baptisms for Mary and James, children of Charles and Ann.
Ann doesn't marry until 1842 and her father is listed as Charles and she appears to be with him in 1841. Charles married twice and on the second marriage his father is listed as Charles as well.
Returning to Charles the father, he marries Ann Dicks in 1795 Tetbury but the first baptism listed (for Mary) doesn't occur until 1804. Not unusual in itself but there are the possible 'unbaptised' children and one possible theory would be that after they married they left the country for some reason, had children including John born at sea and then returned to Tetbury.
The anomalies with the birthplaces for the others could be explained by easy/lazy assumptions/answers made when the censuses were recorded. John may have been the only one who moved from Tetbury who gave their own details.
All speculation though which I can't prove. I looked for other relatives in households that would connect them but couldn't see any.