Running the next query

SET @rownum := 0;    
SELECT customer_id, @rownum := @rownum +1 AS rank, score
FROM game_table_customers gtc
INNER JOIN customers c ON c.customers_id = gtc.customer_id
INNER JOIN game_table gt ON gtc.table_id = gt.table_id
WHERE c.my_team =11095
AND gt.event_id =21110
ORDER BY score DESC
LIMIT 0 , 30

The right result need to look such as this

CustID  Rank    Score
2        1     130000
39       2     99426
84       3     99178
259      4     98963
339      5     97796

However, what I get may be the following

CustID  Rank    Score
2        2      130000
39      11      99426
84      20      99178
259     54      98963
339     69  97796

After I exclude the big event_id clause I recieve the right result. However, if the big eventidentification is incorporated it skews it. I've attempted just the event_id (getting rid of the my_team clause) and same incorrect result.

Any ideas/suggestions why the end result might be off could be much appreciated

I suppose it has related to the execution plan selected in each and every situation and also the mix with using of row amounts - in certain of individuals plans. The row amounts are calculated and so the WHERE the weather is examined, this is why non-consecutive row amounts.

Do this (first obtaining the 30 results you'll need, then using row amounts):

SET @rownum := 0; 
SELECT customer_id, @rownum := @rownum +1 AS rank, score
FROM
  ( SELECT customer_id, score
    FROM game_table_customers gtc
      INNER JOIN customers c ON c.customers_id = gtc.customer_id
      INNER JOIN game_table gt ON gtc.table_id = gt.table_id
    WHERE c.my_team =11095
      AND gt.event_id =21110
    ORDER BY score DESC
    LIMIT 0 , 30
  ) tmp
ORDER BY score DESC

I can not test drive it now but you may also do this:

SELECT customer_id, @rownum := @rownum +1 AS rank, score
FROM game_table_customers gtc
  INNER JOIN customers c ON c.customers_id = gtc.customer_id
  INNER JOIN game_table gt ON gtc.table_id = gt.table_id
  CROSS JOIN (SELECT @rownum := 0 AS rn) dummy  
WHERE c.my_team =11095
  AND gt.event_id =21110
ORDER BY score DESC
LIMIT 0 , 30