Very unusual if they were twins, as presumably they would have been born close to each other in the same place, and that is where their births would be registered. The only scenario I can think of for the difference would be if one was born first at home, there were complications with the second and the mother was taken to a hospital in another borough where the second one was born.
There is a marriage of a Frederick Marshall in 1899 at St Saviour, Southwark and on the same page Alice Maud M Kettle. Possibly Dorothy was their child?