Inadvertently found that the data amounts taken from Mac's Activity Monitor were using binary rather than base-10 for MB and GB (basically does a netstat and divdes bytes by 1024^2 to get MB, 1024^3 for GB). This immediately accounts for it being 4.85% lower than the amount found in Usage Meter.
Just in case anyone was looking.
* Disclaimer: I am a HughesNet customer and not a HughesNet employee. All of my comments are my own and do not necessarily represent HughesNet in any way.