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Link and filter yield different results

I am trying to select a list of "Guardians" only for "Students" with a "School Year Code" 09/10 or 10/11. The "Students" and "Guardians" are stored in the "Customer" table, and the relationships between students and guardians are defined in the "Student Guardian Relationship" table.

I would have anticipated being able to use the following LINK function to return the desired records: (using count instead of rows for testing)
=NL("CountUnique";"Student Guardian Relationship";"Guardian No.";"Link=";"Customer";"No.";"=Student No.";"School Year Code";"09/10|10/11";"Type";"Student")
…but this functions returns all the "Guardians" in the "Student Guardian Relationship" table.

Too check my sanity I used the following filter function:
=NL("CountUnique";"Student Guardian Relationship";"Guardian No.";"Student No.";NL("Filter";"Customer";"No.";"School Year Code";"09/10|10/11";"Type";"Student"))
…this function returns the correct set of records.

This is already a solution, but I would like to improve my understanding of Jet. Can someone explain to me why these to functions returns different results? Thanks in advance.

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