Fortinet 100D report shows wrong bandwidth ?

I get every day scheduled report and bandwidth is something that is strange here. There is 20Mb link on this fortigate and it shows peak of 1.6Mb ? For test i downloaded from one PC some ISO image. Download is going around 1.5MB (so around 12Mb), and i even see that in Link Monitor. But on reports i don'n see that peak of 12Mb (download went for about 30mins).

Any toughs?