Portsea, Hampshire. Macavoy parentage mystery

[Quester]

Hi,
 My 1st cousin three times removed - Dorothea A. Macavoy was born in India about 1872. In the 1891 census she is listed as the daughter of William Robert Macavoy and his wife Helen Augusta nee Oakshott. However, this census gives Dorothea's surname as Thomas. On Ancestry.co.UK a member has put a message next to this census that says - and I quote - "Dorothea has been listed as being the daughter of William & Helen McAvoy. However, she is not related in any way."
There are some records put forward by Ancestry that suggest in the India baptisms she might be Dorothea Amy Laura Thomas born in Secunderabad, Madras, India daughter of Lancelot Francis Charles Thomas and his wife Mary Anne. They also point to the 1881 census in which she is living in Jersey with Mary Anne and some siblings also born in India.
How can I prove whether this is my Dorothea A.Macavoy and if so, why would she be living with a different family in Portsea?
Any light shed on this mystery would be most gratefully received.
Best wishes from Quester

[KGarrad]

If you look at the census image from 1891, there is a single slash (\) between William & Helen and Dorothea.
This denotes a separate family or household within the dwelling.
Dwellings are separated by a double slash (\\).

28 Hampshire Terrace actually contains 3 household groups:
George & Margaret Hackett, a niece and a boarder
William & Helen McAvoy
Dorothea A Thomas

[ciderdrinker]

Hi
There is also a Will for the mother Mary Ann Thomas 16.10.1897 widow of Saxeweimer Road Southsea to Constance Mary Smith wife of Arthur William Smith .£107.

Dorothea Thomas is also on the census in 1881 with her father and sister Beatrice Helen in the Channel Islands Villa Laurens.
His Will is 15.4.1882 for £1100.So it looks like the households were in the process of changing after Lancelot's death when the census was taken.

Ciderdrinker

[Quester]

Hi KGarrad and Ciderdrinker,
Thank you both so much for all your helpful suggestions. I can see now that Dorothea is not the daughter of Helen & William. I wonder why Dorothea is listed as their daughter though in the 1891 census?
Best Wishes Quester