Hello. I don't think it has to do with immigration date. It is strictly the first, middle and last name, SIN and DOB.

What I have always asked from first time filers and specially new immigrants is always always ask them for their SIN card or the SIN paper if they are new commers. You will be surprised (based on my experience) how many times the first and middle name is blank and the entire name is in the last name field. When they go into the working field they tell their employer their first and last names and the T4 shows a different names then the one on their SIN paper.

Would be interesting to know what you find about this particular client, after you have checked his/her SIN paper