Plan Cache: Why are multiple plans cached for this stored procedure in SQL Server

Question

I am trying to gain a better understanding as to how SQL Server caches query plans.

For the following table(Populated with the values 1-500,000):

CREATE TABLE [dbo].[Test] ([ID] INT PRIMARY KEY);

And the following stored procedure:

CREATE PROCEDURE [dbo].[TestGet]
    @ID INT
AS
BEGIN;
    IF(@ID IS NOT NULL) BEGIN;
        SELECT [ID]
        FROM [dbo].[Test]
        WHERE [ID] = @ID;
    END;
    ELSE BEGIN;
        SELECT [ID]
        FROM [dbo].[Test];
    END;
END;

Making the following two calls:

EXEC [dbo].[TestGet] @ID = 1000;
EXEC [dbo].[TestGet] @ID = NULL;

Results in two different plans in the query plan cache:

If the stored procedure were re-written as:

CREATE PROCEDURE [dbo].[TestGet]
    @ID INT
AS
BEGIN;
    SELECT [ID]
    FROM [dbo].[Test]
    WHERE [ID] = @ID OR @ID IS NULL;
END;

And the same two calls were made:

EXEC [dbo].[TestGet] @ID = 1000;
EXEC [dbo].[TestGet] @ID = NULL;

Only a single query_hash, and query_plan_hash are cached:

The calls to the stored procedure have the same shape in both cases. Why does one the first stored procedure and call generate two cached plans, with a different query_hash, and a different query_plan_hash? What criteria is SQL Server using to generate these hashes?