If this is your Agnes then it seems that her husband died and she remarried.
1877
Edward FLEETWOOD, age 46, died SYDNEY, death registered SYDNEY #502
1880
William CLARKE m Agnes FLEETWOOD, marriage registered SYDNEY, #445
However I cannot see death in NSW for an Agnes CLARKE listed with parents John and Margaret, nor a death with no parents listed. Of course, information on death certificates is reliant on the knowledge (or otherwise!) of the informant. Couldn't see any children listed to Agnes and Edward.
I notice that Edward Fleetwood's occupation in the newspaper notice is given as 'mariner' so perhaps children may have been born elsewhere.
Judith