Comparison of using CASE with USER VARIABLES

I have this table where I only want to see AB

ID     CODE       COUNT   
102    AB         9
101    AB         8
100    AC         23    //not important!!!!
99     AB         7
98     AB         6
97     AB         5
96     AB         0

      

        
        
        
      

    

Refers to this

ID    NEWID     CODE       COUNT   
102   102       AB         9
101   101       AB         8
99    100       AB         7
98    99        AB         6
97    98        AB         5
96    97        AB         0

      

        
        
        
      

    

Using

SELECT
t.ID, t.CODE, t.COUNT,
@PREVCOUNT - t.COUNT DIFFERENCE,
@PREVCOUNT := t.COUNT  -- Updates for the next iteration, so it
                       -- must come last! 
FROM
(SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
(SELECT @PREVCOUNT := NULL) _uv;

      

        
        
        
      

    

http://sqlfiddle.com/#!2/e0b8b/36/0

So,

Step 1: 9 - 1 = 1
Step 2: 8 - 7 = 1
Step 3: 7 - 6 = 1
Step 4: 6 - 5 = 1
Step 5: 5 - 0 = 5

      

        
        
        
      

    

Now there may be a case where I have

ID    NEWID     CODE       COUNT   
102   102       AB         4
101   101       AB         2
99    100       AB         1
98    99        AB         0
97    98        AB         7
96    97        AB         0

      

        
        
        
      

    

Then I want to count

Step 1: 4 - 2 = 2
Step 2: 2 - 1 = 1
Step 3: 1 - 0 = 1
Step 4: 0 - 7 = -7      //want  to discard this negative value
Step 5: 5 - 0 = 7

      

        
        
        
      

    

Where I want to abandon step 4 because it is negative.

Now I am using this code to reverse the negative value

SELECT
t.ID, t.CODE, t.COUNT,
@PREVCOUNT,
@PREVCOUNT - t.COUNT DIFFERENCE,
CASE @PREVCOUNT - t.COUNT WHEN @PREVCOUNT - t.COUNT >= 0 THEN 'equal or bigger' ELSE 'smaller' END,
@PREVCOUNT := t.COUNT  -- Updates for the next iteration, so it
                       -- must come last!
FROM
(SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
(SELECT @PREVCOUNT := NULL) _uv;

      

        
        
        
      

    

After running this code, I expect to see where the comparison made is greater than or equal to 0, instead I get very different results.

Below are links to see what I mean:

http://sqlfiddle.com/#!2/6be4a/1

I would really like to know what is wrong here and I would like to have a solution.

respectfully