Usage is measured in 5 minute intervals. There are 300 seconds in each 5 minutes period (60 sec. X 5). The usage is averaged out over these 5 minute or 300 second intervals. The result is your measured use is actually "evened out" or even lower in some cases.
There are 8,000 5 minute intervals in a 30 day period. This means we measure bandwidth usage a total of 8,000 times during the month.
However . . . we take the 400 highest download/upload bursts or 5% of the month's 8,000 - 5 minute measurements and disallow these from the measurement sample. The benefit is that you are only measured on 95% of your total usage during the month. i.e. the "95th Percentile"
If you happen to burst above the set level that 401st time; you are charged either 1 cent or 1 dollar (depending on the type of service) per kbps that you burst above the set level.