Fail to select ALL the datas in C*

[xutom]

Hi all,
   Now we insert 1 billion rows or more of datas into C*,  and then we use 
select command to export datas into local files. but each time when we use such 
SELECT command : SELECT * from table where id = xxx and id2 > value1 and id2 <= 
value2; to query the datas in C* and then export the datas into local files or 
hdfs files, we have diffirent numbers of datas? Can somebody knows the reason？

Thanks,
jerry