Here's a thought.

Given that a binary representation of a value is as unambiguous a way to represent a value as you can get, explain how this occurs:

select convert(varbinary, convert(nvarchar(3),'674')); -- result 0x36003 7 003400

select convert(varbinary, convert(nvarchar(3),'654')); -- result 0x36003 5 003400

Running a checksum on the two binary values gives them a different result from the original checksum and from each other this time (unlike the original where the two were the same). Is this due to the way checksum parses unicode vs. binary?