the difference is simple - unit conversion. Apparently your RADIUS reporting uses base 1024, but Orion family products use base 1000 (see table on the right: Kilobyte - Wikipedia, the free encyclopedia) to convert bytes to KB, MB, GB.
So where NTA shows 1.6 GB transferred, in fact it is 1,600,000,000 B (+- some rounding), the RADIUS displays the same number as 1,600,000,000 B /1024/1024/1024 =1.49 GB. Same for other numbers.
Thanks Choly ! that was also the assumption we made after a lot of pondering but its good to have it validated