The reason is because some people fall into two or more groups. For example, since my dad is European and my mom is Chinese, I would put "Chinese", "English", Scottish", "Irish", "Welsh" and maybe even "Canadian" on the census. So it would look like five or six different people, but in reality, it's just one person.
This is according to the 2011 Canadian census. Ethnicity is self-reported, so 13.2% of people put "Canadian" as their ethnicity. Usually this is people who are of Anglo-descent but have been in Canada for several generations and don't know how much English/Irish/Scottish ancestry they might have. People of other backgrounds sometimes put "Canadian" as well, for example it is the main ethnicity reported by Quebecois, who are mostly French descent but often identify more as "Canadian" since they have lived here for 400 years.