Apologies if this info is not new to you. The birth is also
registered indexed* with the surname Sargent (ref. 2c 37). The mother appears to have been married previously, and the parents not married until 1914.
There is a Sargent - Plaster marriage in Bristol in 1907
and a Starling - Sargent marriage in Devonport 1914. The bride's first name is the same each time
So it looks as if one registration (ref.2c 2) was given as if the parents were a married couple, and the second includes the mother's former married name.
http://discovery.nationalarchives.gov.uk/SearchUI/Details?uri=C8005042*sorry, should have said indexed in the first place. There will only be one registration for that reference, but indexed twice to show the two different names.