Hello,
It is because of receipt and issue movement has been occurred in different date bucket.
Ex: -
Stock rec - 01.10.2014 100 MT
Stock Issue - 15.10.2014 -100 MT
Now if your current date is 20.10.2014
and if you are considering movement quantity then in 0-10 it will show -100MT ( 10.10 to 20.10 record)
and in 10-20 it will show 100 MT ( 01.10 to 10.10 record)
Please let me know if you need any further clarification.
Regards,
Ashish