WIthout knowing what you downloaded and what app was used hard to say.
e.g. some applications use compression which can make 1.6GB actually be 355MB of data transfer across the link.
Hence suggestion to try maybe a 20 MB or 100 MB test file as in the tbbmeter menus. If the meter is a factor of three times out (and under reading) it should be noticeable with those test files.
Are you using Kaspersky Anti Virus, as some versions have created double readings, and the solution worked for that version, but on some systems gave a half sized reading. You can get around this, by Disable TCP/UDP monitoring in the Types of Traffic to Monitor menu item.
There is a download 5MB and record TCP Data Log for that download option, which creates a CSV which can then be used to verify the data calculations.