SQL Server 2008 R2 makes CONVERT_IMPLICIT tinyint for a bit

I am running a simple query on SQL Server 2008 R2 for the following table:

Id (int, not null)
Enabled (bit, but not null)

Both columns have separate indexes.

So when I run the following query:

SELECT Id FROM Entities WHERE Enabled = 1

      

        
        
        
      

    

The execution plan shows that INDEX_SCAN is executing (it is called by CONVERT_IMPLICIT on the Enabled column)

And when I run another request:

SELECT Id FROM Entities WHERE Enabled = '1'

      

        
        
        
      

    

or

SELECT Id FROM Entities WHERE Enabled = 'true'

      

        
        
        
      

    

or

SELECT Id FROM Entities WHERE Enabled = CAST(1 AS BIT)

      

        
        
        
      

    

The execution plan shows that INDEX_SEEK is done.

How CONVERT_IMPLICIT can affect performance on more complex queries, I want to know what is causing SQL Server to behave this way?

UPD:

If I run

SELECT Id FROM Entities WHERE Enabled = 0

      

        
        
        
      

    

and then

SELECT Id FROM Entities WHERE Enabled = 1

      

        
        
        
      

    

The execution plan shows INDEX_SEEK. In this case, I think SQL Server has collected some optimization statistics and finally found out that there is no reason for CONVERT_IMPLICIT. But unfortunately, I cannot guarantee that my original request will execute with the opposite value.

I would appreciate any clarification I can get.