Grouping Columns In A Varaiable When Using PIVOT?

Hi,
I just read on web that we can not use grouping columns in a variable when using PIVOT operator. For example like,
USE AdventureWorks
GO
SELECT VendorID, [164] AS Emp1, [198] AS Emp2, [223] AS Emp3, [231] AS Emp4, [233] AS Emp5
FROM
(SELECT PurchaseOrderID, EmployeeID, VendorID
FROM Purchasing.PurchaseOrderHeader) p
PIVOT
(
COUNT (PurchaseOrderID)
FOR EmployeeID IN
( [164], [198], [223], [231], [233] ) // cannot put these in a variable like @Col
) AS pvt
ORDER BY VendorID;



Though it can be achieved using when making the query using dynamic sql. If some can make it clear why it is possible using dynamic sql and not with the above code.

Regards,