Why do the English pronounce the first name "St. John" as "sinjin" but they pronounce the last name "Saintclair" as "saint clair" (and not "sinclair")?

In Vancouver, Canada, we have a journalist named St. John Alexander who pronounces his first name as "Sinjin." I heard him say it on TV and it sounded weird. His profile even mentions it.

He's often asked about his name. St. John is originally British and is pronounced "Sinjin." His parents discovered it in Charlotte Bronte's novel Jane Eyre.

From Wikipedia St John Pettifor Catchpool (1890–1971), English Quaker relief worker St. John Ellis (1964–2005), British Rugby League player St John Ervine (1883-1971), Irish writer St John Groser (1890-1966), Anglican priest and Christian socialist St John Hornby (1867–1946), British businessman St John Horsfall (1910-1949), British motor racing driver St John Brodrick, 1st Earl of Midleton (1856–1942), British politician St John O'Neill (1741–1790), Irish MP for Randalstown Saint-John Perse, pseudonym of Alexis Leger (1887–1975), French poet and diplomat St John Philby (1885–1960), British civil servant and explorer in Arabia