Perhaps it is. Newer drivers are limited to 4 cards in Win7 and 5 cards in Win8. I don't care where the limitation currently resides - the driver has those limits on those environments. I have dedicated mining systems running each OS and have seen the issues firsthand. I'd use Linux if I didn't already have a custom setup for Windows that allows me to instantaneously update all my miners from my phone (send three characters and they respond to my new settings/new mining app files/etc). 

I'm sure you will agree with the information available online for the OP's motherboards, the limitation does not appear to be motherboard. He needs to use a different version. If he wants five cards working, he would need Windows 8 if he wants to run the latest stock drivers.