About Count of missing vaule, Count Distinct

Hi,

Can anybody tell me why this happens for these pass through SQL?

SUM(CASE WHEN var1 IS NULL THEN 1 ELSE 0) as cnt_missing FROM table

generates a value of 2, which is false.

COUNT(*) AS cnt_missing FROM table WHEN var1 IS NULL

generates a value of millions, which is true.

COUNT(DISTINCT var2) AS cnt_unique FROM table

generates a value of 2, which is false

COUNT(*) AS cnt_unique FROM (SELECT DISTINCT var2 FROM table)

generates a value about 3,000, which is true.

Thanks.