I discovered that VF round up your usage to the nearest 10kB at the end of each 'data session' or every 20 minutes, whichever comes sooner. It's in the Terms and Conditions.
The 20 minutes is significant - If you are continuously connected to mobile data, there's 2160 20-minute periods in a month (that's a 30-day month). If you get rounded up 5kB on average, that's an extra 11MB of data on your bill per month that you did not actually use. If your data sessions frequently stop and start (It never defines what a 'data session' is), it could be a lot worse.
VF Australia only round in 1kB increments, which is 10 times better for users. Why is it 10kB in NZ?