That would be because there is traffic processing and classifying in this software. This requires CPU time to handle the data, which may reduce the maximum speed you are getting.
Is your connection really 1 Tbps? Most business and home Internet connections are capped at 1 Gbps.
Sorry, you are right, it's 1Gb. But so do I understand it right that if I download something for a longer time, the program should show the correct transfer?