There are many, many Mary Bryans who are known as Polly Bryan and Mary Ann Bryans who are known as Polly Ann Bryan. The same thing happens with other surnames as well. The following is the first explanation I have seen of this phenomenon.
First Hypertext Edition of the Dictionary of Phrase and Fable
THE DICTIONARY OF PHRASE AND FABLE BY E. COBHAM BREWER
FROM THE NEW AND ENLARGED EDITION OF 1894
Polly Mary. The change of M for P in pet names is by no means rare; e.g. - Margaret, Maggie, or Meggy, becomes Peggie, and Pegg or Peg. Martha. Matty becomes Patty. Mary. Molly becomes Polly or Poll. Here we see another change by no means unusual- that of r into l or ll. Similarly, Sarah becomes Sally; Dorothea, Dora, becomes Dolly; Harry, Hal.