Defense score...

Why does my slave country with have half the weaponry of my main have a defense score 10 points higher than my main. For example my main has 100 fortifications slave 73 main 6 airports 5 defense bases slave 4 and 3 respectively main 289 radar planes slave 20 main 10,905 helos 10,806 interceptors slave 1700 helos 1600 intercepters and it seems the more gear I buy for my main the lower the def score goes...WTF is going on...