http://en.wikipedia.org/wiki/Social_Insurance_Number
The first digit of a SIN indicates the province in which it was registered:
1: Atlantic Provinces: Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland and Labrador
2: Quebec
3: Quebec
4: Ontario (including overseas forces)
5: Ontario
6: Prairie Provinces (Manitoba, Saskatchewan, and Alberta), Northwest Territories, and Nunavut
7: Pacific Region (British Columbia and Yukon)
8: Not used
9: Temporary resident
0: Not used (Canada Revenue may assign fictitious SIN numbers beginning with zero to taxpayers which do not have SINs)
つまり9で始まるのはテンポラリー(ワーホリ等)、7は普通は西海岸で登録されていると言う意味で移民は関係ありません。
Note: While the first digit usually identifies the location of registration, the government has found it necessary in the past to supply certain regions with SIN numbers assigned to other regions.
つまりこの場合、ワーホリだと普段は9で始まるナンバーですが何らかの理由で「7」ナンバーを振り当てたんでは?(「9」ナンバーが足りないとか。実際日本からのワーホリだけで年間5000個割り当ててる計算になりますし、他所から来てる人も沢山いるでしょうし)