在SQL Server 2012中使用FIRST_VALUE（）时出现意外结果

When I use FIRST_VALUE on a data set that I construct by hand I get one result, and when I use it on a data set that results from a left join, I get a different result - even though the data sets appear to me to contain the exact same data values. I've reproduced the issue with a simple data set below.When I use FIRST_VALUE on a data set that I con