Deployment Pay question?

My husband left for Iraq a month ago, is an e-3, and his second paycheck after he left was significantly higher than the amount he had been getting while he was still stationed in the states. But then his paycheck now(2 weeks after), is a different amount again. It is more than the pay he was getting while here, but lower than the amount that he received in the last paycheck. Is there a reason behind this? is this normal?