Data usage discrepancy

I have a simcard used exclusively for data in a router. The router shows 3.237 GB used while MyVodacom shows 4.43 GB used over the last 8 days. Surely there cannot be a 37% difference in measuring the exact same usage? Who is taking who for a ride? Unfortunately judging by their business ethics, I tend to believe that Vodacom is making my data balance disappear faster than I am using it...