Confusing Dates

I thought that I would write today about how difficult it can be to ascertain certain dates when looking at records. The following is for Henrietta Westhoff (1907-1994) who is Dermot's 1st cousin twice removed.

Their common ancestors are John Jacob Westhoff (1831-1908) and Mary Ann Mooney (1825-1918), who were Dermot's 2nd great grandparents and the grandparents of Henrietta.

Henrietta was the 8th child of Henry Westhoff (1869-1920) and Johanna Fleming (1871-1954). 

She was born on and this is where it gets tricky as you can see from the following.

I do not have a copy of her actual birth record, but her birth was registered for the 3rd quarter of 1907 in the General Register Office (Vol 8B, p31). 

On the 1911 census her age is given as 3, which would make her year of birth as 1907 (the census was taken on 2 April 1911).

Henrietta married James Joseph Coggins in 1926 and she is on the 1939 register, with her date of birth given as 30 June 1907. 

However, on her death registration in 1994, her date of birth is given as 29 June 1902. I have also found a death notice, which gives her age as 91, which would make her date of birth 1902.

From the Liverpool Echo 10 March 1994, page 23

I believe that her correct year of birth is the one that is given on the birth registration, the 1911 census and 1939 register, which is 1907. I wonder when and how it got changed?