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  • Dustin Waybright thinks this is interesting:

When comparing the NULLs in the region column in the rows from the two sides, the INTERSECT operator considered them as equal.

From

Cover of Training Kit (Exam 70-461): Querying Microsoft® SQL Server® 2012

Note

This is necessary or matching rows with the same NULL columns would be eliminated. This is different than the WHERE and ON clauses.