If Burger King couldn't bill an average of 3% of their customers due to billing error, they'd raise their prices 3%. The net amount paid by their customers would still be the same and their total revenue would still be the same. They'd still be just as competitive. They'd just be billing based upon, you guessed it, statistical sampling.
Even if I lose 3% of all flows that does not mean that I also lose 3% of valuable data. It depends on which flows have been thrown away. In the worst case you may lose nearly 100%, in the best case you almost lose nothing. It would be interesting which algorithm is been chosen for throwing away flows.
Logic would dictate that during the heaviest traffic, when the router is working the hardest, it's more likely to lose flows. But that's just what you'd expect, not necessarily what actually happens.
Furthermore: you will never bill byte by byte. That means a customer has to pay x $ per Gig. If he used 2.1 Gig he has to pay for 3 Gig, if he used 2.9 Gig he also pays for 3 Gig.
In other words, a customer that uses 2.99 gig pays less than a customer that uses 3.00 gigs. So that 3% loss could make a big difference. Yes, it will make a difference less often, but when it does, it will be a bigger difference. In fact, I would expect that a 3% loss will, on average, result in 3% less revenue from overusage. Again, the billing is based upon statistical sampling. Really. DS