Darkstat traffic not counted correctly

I have run the same test on several different computers plus myNanoPI r5s , and everytime
I download a test ubuntu-desktop image darkstat count less bytes than iftop and other programs.

I have run the darkstat with the following cmdline.

darkstat -p 668 -i br-lan -i eth0 -i eth1 -i eth2 -i vethDeMKL5
darkstat -p 668 -i br-lan
darkstat -p 668 -i eth0 

and with the -l command too.

the ubuntu test image is ca 3.6 GB, and this is was darkstat shows , when I enable all the network interfaces.

Thanks Michael.