An unusual number of American surnames that begin with ‘R’ end in ‘S’. Why is this?

Data: Of the 500 most common American surnames, 34 begin with ‘R’. Of these, –

12 end in ‘S’ (Roberts, Reeves, etc.)

6 others end in an ‘S’ sound (Ruiz, Reese, etc.)

The remaining 16 end in other letters or sounds.

[Source: 'Frequently Occurring Surnames in Census 1990' at www.census.gov]