Interesting question, thanks, and a good reason you should always specify the data type for hardcoded items when using a Table Value Constructor or UNION since data type precedence comes into play.

Here is a link to the Table Value Constructor in BOL that mentions the data type precedence: http://msdn.microsoft.com/en-us/library/dd776382.aspx, and here is a link for data type precedence: http://msdn.microsoft.com/en-us/library/ms190309.aspx

Edited to correct the information. (thanks Bradley!)