Warning in query plan “Cardinality Estimate”

Question

create table T(ID int identity primary key)
insert into T default values
insert into T default values

go

select cast(ID as varchar(10)) as ID
from T
where ID = 1

The query above has a warning in the query plan.

<Warnings>
  <PlanAffectingConvert ConvertIssue="Cardinality Estimate" Expression="CONVERT(varchar(10),[xx].[dbo].[T].[ID],0)" />
</Warnings>

Why does it have the warning?

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