10774 - merge join vs. NESTED LOOP JOIN

i execute this query
SELECT soh.*
FROM Sales.SalesOrderHeader soh JOIN Sales.SalesOrderDetail sod
      ON soh.SalesOrderID = sod.SalesOrderID
in the execution plan see MERGE JOIN

when execute
SELECT soh.*
FROM Sales.SalesOrderHeader soh JOIN Sales.SalesOrderDetail sod
      ON soh.SalesOrderID = sod.SalesOrderID
WHERE soh.SalesOrderID = 71832
in the execution plan see NESTED LOOP JOIN

why?