Ismail,
System wont create both WIP and variance for a single order at a time, as per your query above after goods issue, if you make your order status as TECO and DLV for the first period itself variance got calculated and nothing to calculate in the next month, if you calculates the WIP in the first period and if you change your order status in the next month, then system will reverse the previous period WIP and post the variance.
I am not sure why system calculated both WIP and variance in your case, to cross check i executed the same scenario from my end, but system behavior is as explained above.
Regards,
Ravi