Warning in query plan "Cardinality Estimate"
This warning was new for SQL Server 2012.

From
New "Type Conversion in Expression....." warning in SQL2012 ,to noisy to practical use


  I see what you mean. While I agree that this is noise in most cases,
  it is low priority for us to fix. We will look at it if we get more
  feedback. For now I have closed this by design.


Connect was killed and it doesn’t look like the original issue was transferred to UserVoice. Here’s a different UserVoice issue about the same problem, Type conversion in may affect CardinalityEstimate - Convert/cast on selected columns

I will provide the boring answer until someone comes along with a better one.


  Why does it have the warning?


Speculation on my part.
There is a cast on a column that is used in the where clause which make statistics of that column interesting. A change of datatype makes the statistics no good so lets warn about that in case the value from the field list might end up to be used somewhere.


  How could a cast in the field list affect the cardinality estimate?


It can't unless it is the field list in a derived table.