Thanks to

The advisor show us that we can get different behavior when we use condition , ='20000' or N'20000'

CODE
/*+++++++++++++++++*/
set nocount on
go
create table dbo.Data
(
Id int not null identity(1,1),
VarcharColumn varchar(10) not null,
Placeholder char(200) null
constraint DEF_Data_Placeholder
default 'The placeholder',
constraint PK_Data
primary key clustered(ID)
)
go

declare
@I int
select @I = 0
begin tran
while @I < 50000
begin
insert into dbo.Data(VarcharColumn)
values(convert(varchar(10),@I))
select @I += 1
end
commit
go
create unique nonclustered index IDX_Data_VarcharColumn
on dbo.Data(VarcharColumn)
go

/*+++++++++++++++++*/

In his article, He means

However, as my site enviroument ,

Microsoft SQL Server 2014 - 12.0.2000.8 (X64) Enterprise Edition (64-bit) on Windows NT 6.1

I get an different result

 and

 Maybe in sqlserver new version . the SQL engine enhanced.