Because the census is utterly clueless on how to properly classify people. The Canadian system is far superior.
How's the Canadian system?
It's not perfect either, but:
The census (actually, the "National Household Survey" [NHS] which is an optional form that 1/3 of Canadians get along with the mandatory census) asks whether or not you're a visible minority. If you're not a visible minority you're either White or Aboriginal. (because I guess calling the original inhabitants of this country minorities is offensive?)
If you are a visible minority, you may select one the following options:
South Asian
Chinese
Black
Filipino
Latin American
Arab
Southeast Asian
West Asian
Korean
Japanese
Visible minority, n.i.e. (write ins)
Multiple visible minorities
Also, the NHS has a separate question asking about ethnicity.
