Wrong result from distinct and arithmetic expression using window function

WITH tab(t, company, quote) AS (

  SELECT 1 AS t,   'G' AS company, 40 AS quote

  UNION SELECT 2 , 'G',    60 

  UNION SELECT 3 , 'S',    60 

  UNION SELECT 4,  'S',    20

)

SELECT DISTINCT company, quote - LAG(quote) OVER(PARTITION BY company ORDER BY t)

FROM tab;

company	e

--------------

G	(null)

S	(null)

WITH tab(t, company, quote) AS (

  SELECT 1 AS t,   'G' AS company, 40 AS quote

  UNION SELECT 2 , 'G',    60 

  UNION SELECT 3 , 'S',    60 

  UNION SELECT 4,  'S',    20

)

SELECT company, quote - LAG(quote) OVER(PARTITION BY company ORDER BY t) e

FROM tab;

company	e

--------------

G	(null)

G	20

S	(null)

S	-40