Could I get some help with my function and query?

I wrote a function that calculates the discount price of an item:

IF OBJECT_ID('fnDiscountPrice') IS NOT NULL
	DROP FUNCTION fnDiscountPrice;

GO

CREATE FUNCTION fnDiscountPrice
	(@ItemID	int	= 0)
	RETURNS money
AS
BEGIN
	DECLARE @DiscountPrice money
	SET @DiscountPrice = (SELECT (ItemPrice - DiscountAmount) FROM OrderItems WHERE ItemID = @ItemID);
	RETURN @DiscountPrice
END

That operates as it should, although I am having difficulties using it properly within a query. I have tried it in two ways - both below. The take that the function is iterating through has 12 rows of data, therefore the query should return 12 results, however it returns 200+.

First attempt:

DECLARE @count int = 1;
BEGIN
WHILE (@count < (SELECT Count(ItemID) FROM OrderItems))
	SELECT ItemID, ItemPrice, DiscountAmount, dbo.fnDiscountPrice(@count) AS DiscountPrice
	FROM OrderItems
	SET @count = @count + 1
END

Second attempt using a temporary table:

DECLARE @count int = 1;
WHILE (@count < (SELECT Count(ItemID) FROM OrderItems))
BEGIN
	INSERT INTO #Temp
	SELECT ItemID, ItemPrice, DiscountAmount, dbo.fnDiscountPrice(@count) AS DiscountPrice
	FROM OrderItems
	WHERE ItemID = @count
	SET @count = @count + 1
END

SELECT * FROM #Temp

As you can see they are very similiar, I just don't understand why I am getting the results I am.

Any insight you could provide would be both helpful and appreciated.