Netflow shows double the bandwidth on link

Hello,

We're installing Cisco 4331 series routers at a couple of our remote sites.  Each Router has 8 T1s in a Multilink.  In NPM the available bandwidth shows 12.288Mbps.  When I do stress testing NPM shows utilization is 12.288Mbps.  When I check Netflow Traffic Analyzer during the stress test it shows utilization of 23.9Mbps.  I am currently at a loss as to why there is a difference with NTA.  Anyone have any idea what could be happening?

Thank you