How Can NULL Be Equal To Today's Date (was Weird Query Behavior Plz Help)

I have a query that is behaving a little a weird. here is the example:

i have 1 table in this table i have 2 columns wich are date and time

DATE_DEBUT_PERIODE_FISCALE DATE_FIN_PERIODE_FISCALE
------------------------------ ---------------------------
1/27/1997 2/27/1997
1/1/2005 2/6/2005

here is my query:

BEGIN
declare @datefin_flag datetime, @strip datetime
SELECT @strip = dateadd(d,datediff(d,0,getdate()),0)
SELECT @datefin_flag = DATE_FIN_PERIODE_FISCALE FROM DM_LKP_CALENDRIER_PERIODE_F
WHERE DATE_DEBUT_PERIODE_FISCALE < @strip AND DATE_FIN_PERIODE_FISCALE = @strip
--select @datefin_flag
--select @strip
IF(@datefin_flag != @strip)
RAISERROR('You cant run this',16,1)
END

Well this Query should return the raiserror it returns completes successfuly
since todays date is not the same as the date in the database.
if you select @datefin_flag it returns NULL and if you select @strip it brings back todays date how can NULL be equal to to todays date assuming that todays date is equal to NULL. ?