Please Explain This

Would someone care to try and explain this? I just received a check from my insurance company for forty eight dollars and change as an over payment after having had the policy which costs three sixty something for six months of insurance on my truck. for about a month. I have never had an at fault accident in seventeen years of driving and have a perfect driving record, not even a speeding ticket ever at the age of thirty three. I have added my wife to this policy, she was out of town when I bought the truck and didn’t have her license on me so I couldn’t add her to the policy at the time. This is our only vehicle mind you. She has had her license for going on two years at the age of thirty one. I am not complaining about the refund but I can’t quite figure out how or why the insurance should go down thirteen percent for adding a woman to the policy, especially one who hasn’t been driving for a fraction of the time I have had a perfect driving record. Anyone want to explain this?