Hi
What Ajay sir told is 100% correct.
I can only guess that you have some fixed cost involved in your activity and the activity output is lower in second period as compared to first period so the fixed charges are inflated. Kindly check and revert.
Second guess is that in valuation variant (used in costing variant for std cost), for activity valuation you have used actual price for last period and for any reason your actual price for last period is very high. Sounds weird... 
Regards
Rajneesh