Strange Behavior of NULL Values in SQL Queries

I found a strange behavior of NULL Values when I have a table values as in the following:

Declare @val table
(
Id int,
Username varchar(100),
Desg varchar(100),
Level varchar(100)
) 

Insert into @val
select 1,'AAA',NULL,1 union all
select 1,'BBB',NULL,1 union all
select NULL,'CCC','SE',1 union all
select 2,NULL,'PL',4 union all
select 2,'EEE',NULL,5

-- Query 1 
select * from @val  

-- Query 2 
select Id,Username,desg,COUNT(*)
[No. of  Employees] from @val
where desg  <> 'SE'
group by Id,Username,Desg

-- Query 3
select Id,Username,desg,COUNT(*)
[No. of Employees] from @val
where desg is null or desg  <> 'SE'
group by Id,Username,Desg

When we execute the above queries in the Sql Server Management Studio, we would get the following outputs:

Query 1 Result:

This is the output of the entire table we just created.

Query 2 Result:

This is the output when I use this condition – desg <> ‘SE’. But it should show the remaining rows that are with NULL values.

Query 3 Result:

when I use this condition – desg IS NULL or desg <> ‘SE’, I am able to get the required output.

Somebody who knows can explain the trick ;-)