Pels told me about a Brock marriage I had missed (Thanks again, Pels!)

I was intrigued and bought the certificate. Now I am confused!

The certificate (21 March 1869, Alburgh) has the following information:
John Brock, full age, Widower, Carpenter, resident in Alburgh, Father John Brock, Carpenter
Frances Reed, Full age, Widow, no prof., resident in Alburgh, Father William Wall, Labourer
(Witnesses S Y Brock and Kate Brock)
Here they are in 1871:
Alburgh Census
John Brock, 60, Carpenter, b. Gt Yarmouth
Frances, 59, Dressmaker, b. Denton
and in 1881
Alburgh Census
John Brock, 70, Carpenter out of employ b. Yarmouth
Frances, 70, b. Denton
But where was John before then?
Well, there is:
1851 Alburgh
John Brock, 40, Carpenter Journeyman b. Alburgh
Elizabeth 40, b. Roydon (?)
Ellen, 8 b. Alburgh
Susanna, 3 b. Alburgh
Mary Ann, 1 b. Alburgh
1861 Alburgh
John Brock, 51, Carpenter b. Alburgh
Elizabeth, 51, b. Raydon
Ellen 19, b. Alburgh
Susanna 13, b. Alburgh
Mary Ann 11 b. Alburgh
But this one is born in Yarmouth, not Alburgh.
And there is also:
1841, Alburgh:
John Brock, 30, Ag Lab b. Norfolk
Elizabeth, 30
Infant (F), 4 days (aaah!)
and
1841, Gt Yarmouth, Row 36
John Brock, 30, Sawyer b. Norfolk
George, 15
William,11
Edward, 9
I can't find any others.
Did he just change his birthplace? Why? how many John Brocks b. 1811 are we talking about?
meles