My thoughts....
1841 census has a fancy F and tiny n in the righthand column which should mean foreighn parts and
Fanny is recorded as b. Montreal in 1851. Her parents would presumably have given this info to the enumerators and not her personally and one would assume they should know. If noone told her she was born in Canada or she didn't tell her hubby who presumably gave later census info and assumed a local birth, etc., she was thus recorded as b. England. Also, if she was b. in Canada, Fanny would not have remembered ever being there, having come to England at such a young age.
The only record of a birth in Montreal in that time frame would be a baptism and if she was baptized at age 2 it doesn't look like she would have been baptized in Canada. I don't see a Quebec record at the moment.
Anyway, food for thought. Interesting puzzle.
PB